feat: Phase 3 event handler wiring and DataBind pattern conversion (GAP-09, GAP-32)

Add two critical L1 script transforms that eliminate ~70% of remaining manual migration work:

Event Handler Wiring (GAP-32):
- Convert-EventHandlerWiring adds @ prefix to On* event attributes in markup
- OnClick="Handler"  OnClick="@handler" for proper Blazor event binding
- Handles all Web Forms event attributes (OnClick, OnTextChanged, OnRowCommand, etc.)
- Regex-safe: only targets bare C# identifiers, skips existing @ expressions

DataBind Pattern Conversion (GAP-09):
- Convert-DataBindPattern transforms DataSource/DataBind() in code-behind
- Get-DataBindMap pre-scans code-behind for cross-file markup correlation
- Add-DataBindItemsAttribute injects Items="@_fieldName" on matching markup tags
- Generates private IEnumerable<object> backing fields with _controlNameData naming

Tests: 25/25 L1 tests pass (100%), 618/618 lines match
- TC22: DataBind with GridView
- TC23: DataBind with multiple controls (GridView + Repeater)
- TC24: Event wiring with multiple event types
- TC25: Combined DataBind + event wiring
- TC20/TC21 updated for @ prefix in expected output

Docs: Phase3-EventHandlerWiring.md, Phase3-DataBindConversion.md
Skills: CODE-TRANSFORMS.md updated with Phase 3 section
Nav: mkdocs.yml updated with Phase 3 section

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: csharpfritz

docs/Migration/Phase3-DataBindConversion.md

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+# DataBind Pattern Conversion
+
+The migration script automatically converts the Web Forms `DataSource`/`DataBind()` pattern to Blazor-compatible data binding using backing fields and the `Items` parameter. This handles the most common data binding pattern in Web Forms applications.
+
+## Overview
+
+**What it does:**
+- Detects `controlName.DataSource = expression;` in code-behind
+- Removes `controlName.DataBind();` calls
+- Generates a private backing field (`_controlNameData`)
+- Replaces the DataSource assignment with a field assignment
+- Injects `Items="@_controlNameData"` on the matching markup tag
+
+**Why it matters:**
+In Web Forms, every data-bound control requires `DataSource = data; DataBind();` to display data. In Blazor, BWFC components use an `Items` parameter for reactive binding. This transform bridges the gap automatically, eliminating one of the most tedious manual migration steps.
+
+## The Transform
+
+### Before (Web Forms)
+
+=== "Markup (.aspx)"
+    ```html
+    <asp:GridView ID="gvProducts" runat="server" AutoGenerateColumns="false">
+        <Columns>
+            <asp:BoundField DataField="Name" HeaderText="Product Name" />
+            <asp:BoundField DataField="Price" HeaderText="Price" />
+        </Columns>
+    </asp:GridView>
+    ```
+
+=== "Code-Behind (.aspx.cs)"
+    ```csharp
+    protected void Page_Load(object sender, EventArgs e)
+    {
+        if (!IsPostBack)
+        {
+            gvProducts.DataSource = GetProducts();
+            gvProducts.DataBind();
+        }
+    }
+    ```
+
+### After (Blazor — Automated)
+
+=== "Markup (.razor)"
+    ```html
+    <GridView id="gvProducts" Items="@_gvProductsData" AutoGenerateColumns="false">
+        <Columns>
+            <BoundField DataField="Name" HeaderText="Product Name" />
+            <BoundField DataField="Price" HeaderText="Price" />
+        </Columns>
+    </GridView>
+    ```
+
+=== "Code-Behind (.razor.cs)"
+    ```csharp
+    // Data binding fields (generated by L1 migration)
+    private IEnumerable<object> _gvProductsData;
+
+    protected override async Task OnInitializedAsync()
+    {
+        await base.OnInitializedAsync();
+        _gvProductsData = GetProducts();
+    }
+    ```
+
+## How It Works
+
+The transform operates in two phases with cross-file correlation:
+
+### Phase 1: Pre-Scan Code-Behind
+
+Before processing the markup, the script scans the code-behind for `DataSource` assignments:
+
+```
+gvProducts.DataSource = ...  →  Map: { gvProducts → _gvProductsData }
+rptItems.DataSource = ...    →  Map: { rptItems → _rptItemsData }
+```
+
+### Phase 2: Apply Transforms
+
+1. **Markup:** For each control ID in the map, finds `id="controlId"` and injects `Items="@_fieldName"`
+2. **Code-behind:** Replaces `controlName.DataSource = expr;` with `_fieldNameData = expr;`
+3. **Code-behind:** Removes all `controlName.DataBind();` calls
+4. **Code-behind:** Generates `private IEnumerable<object> _fieldNameData;` declarations
+
+## Multiple Controls
+
+The transform handles multiple data-bound controls in the same page:
+
+```csharp
+// Web Forms
+gvOrders.DataSource = GetOrders();
+gvOrders.DataBind();
+rptCategories.DataSource = GetCategories();
+rptCategories.DataBind();
+
+// Blazor (migrated)
+private IEnumerable<object> _gvOrdersData;
+private IEnumerable<object> _rptCategoriesData;
+
+// In OnInitializedAsync:
+_gvOrdersData = GetOrders();
+_rptCategoriesData = GetCategories();
+```
+
+## Field Naming Convention
+
+The generated field name follows this pattern:
+
+```
+controlName → _controlNameData
+```
+
+| Control ID | Generated Field |
+|-----------|----------------|
+| `gvProducts` | `_gvProductsData` |
+| `rptCategories` | `_rptCategoriesData` |
+| `dlItems` | `_dlItemsData` |
+| `lvOrders` | `_lvOrdersData` |
+
+The first character is lowercased and `Data` is appended, with a `_` prefix for private fields.
+
+## Manual Review Checklist
+
+After the automated migration, review the following:
+
+### 1. Typed Data Binding
+
+The generated field uses `IEnumerable<object>`. For strongly-typed binding, update the field type:
+
+```csharp
+// Generated (generic)
+private IEnumerable<object> _gvProductsData;
+
+// Improved (typed)
+private IEnumerable<Product> _gvProductsData;
+```
+
+### 2. Re-binding in Event Handlers
+
+If your code re-binds data in event handlers (e.g., after a delete), the DataSource assignment is also converted:
+
+```csharp
+// Web Forms
+protected void Delete_Click(object sender, EventArgs e)
+{
+    DeleteProduct(selectedId);
+    gvProducts.DataSource = GetProducts();
+    gvProducts.DataBind();
+}
+
+// Blazor (migrated)
+protected void Delete_Click()
+{
+    DeleteProduct(selectedId);
+    _gvProductsData = GetProducts();  // Re-assignment triggers re-render
+}
+```
+
+In Blazor, assigning to the backing field and calling `StateHasChanged()` (automatic in event handlers) triggers a re-render with the new data.
+
+### 3. DataSourceID References
+
+If your control uses `DataSourceID` instead of code-behind `DataSource`, the transform doesn't apply. `DataSourceID` is handled separately with TODO comments — see the [DataSourceID Migration](DeferredControls.md) guide.
+
+## Test Coverage
+
+- **TC22** — Single GridView with DataBind in Page_Load
+- **TC23** — Multiple controls (GridView + Repeater) with DataBind
+- **TC25** — Combined DataBind + event handler wiring on same page
+
+## See Also
+
+- [Event Handler Wiring](Phase3-EventHandlerWiring.md) — Markup event attribute transform
+- [Lifecycle Transforms](Phase2-LifecycleTransforms.md) — Page_Load → OnInitializedAsync
+- [Automated Migration Guide](AutomatedMigration.md) — Full list of automated transforms

docs/Migration/Phase3-EventHandlerWiring.md

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+# Event Handler Wiring Migration
+
+The migration script automatically adds the `@` prefix to event handler attributes in markup, enabling proper Blazor event binding. This transforms `OnClick="Handler"` into `OnClick="@Handler"` so the Razor compiler can resolve the method reference.
+
+## Overview
+
+**What it does:**
+- Adds `@` prefix to event handler attribute values in converted markup
+- Targets `On[A-Z]*` attributes with bare method name values
+- Skips attributes that already have `@` or contain non-identifier characters
+
+**Why it matters:**
+In Web Forms, `OnClick="Button_Click"` in markup is a string that the runtime resolves at page compile time. In Blazor, the `@` prefix is required to tell the Razor compiler to treat the value as a C# expression (a method group), not a string literal. Without `@`, you get compilation errors.
+
+## The Transform
+
+### Before (Web Forms)
+
+```html
+<asp:Button ID="btnSave" Text="Save" OnClick="Save_Click" runat="server" />
+<asp:DropDownList ID="ddlCategory" OnSelectedIndexChanged="Category_Changed" runat="server" />
+<asp:GridView ID="gvItems" OnRowCommand="Grid_RowCommand" runat="server" />
+```
+
+### After (Blazor — Automated)
+
+```html
+<Button id="btnSave" Text="Save" OnClick="@Save_Click" />
+<DropDownList id="ddlCategory" OnSelectedIndexChanged="@Category_Changed" />
+<GridView id="gvItems" OnRowCommand="@Grid_RowCommand" />
+```
+
+## Supported Event Attributes
+
+The transform handles all Web Forms event attributes that follow the `On[A-Z]*` naming convention:
+
+| Event Attribute | Controls | Description |
+|----------------|----------|-------------|
+| `OnClick` | Button, LinkButton, ImageButton | Click handler |
+| `OnTextChanged` | TextBox | Text change handler |
+| `OnSelectedIndexChanged` | DropDownList, ListBox, RadioButtonList, CheckBoxList | Selection change |
+| `OnCheckedChanged` | CheckBox, RadioButton | Check state change |
+| `OnRowCommand` | GridView | Row command handler |
+| `OnRowEditing` | GridView | Row edit begin |
+| `OnRowUpdating` | GridView | Row update handler |
+| `OnRowDeleting` | GridView | Row delete handler |
+| `OnRowCancelingEdit` | GridView | Row edit cancel |
+| `OnPageIndexChanging` | GridView | Paging handler |
+| `OnSorting` | GridView | Sort handler |
+| `OnItemCommand` | Repeater, DataList | Item command handler |
+
+## Combined with Signature Transforms
+
+This markup-side transform works together with the [Event Handler Signatures](Phase2-EventHandlerSignatures.md) code-behind transform. Together they handle the full event migration:
+
+1. **Markup:** `OnClick="Save_Click"` → `OnClick="@Save_Click"` (this transform)
+2. **Code-behind:** `protected void Save_Click(object sender, EventArgs e)` → `protected void Save_Click()` (signature transform)
+
+## How the Script Detects Event Handlers
+
+The regex pattern `(On[A-Z]\w+)="([A-Za-z_]\w*)"` matches:
+
+- **Attribute name** starts with `On` followed by an uppercase letter (e.g., `OnClick`, `OnRowCommand`)
+- **Attribute value** is a bare C# identifier (e.g., `Save_Click`, `Grid_RowCommand`)
+
+Values that are **not** transformed:
+- Already prefixed: `OnClick="@Save_Click"` — no change
+- String values: `OnClick="javascript:doSomething()"` — contains `:()`, not an identifier
+- Empty values: `OnClick=""` — nothing to prefix
+- Expressions: `OnClick="@(() => HandleClick())"` — starts with `@`
+
+## Test Coverage
+
+- **TC20** — Standard event handlers (OnClick on Button, LinkButton)
+- **TC21** — Specialized event handlers (OnRowCommand, OnPageIndexChanging on GridView)
+- **TC24** — Multiple event types (OnTextChanged, OnSelectedIndexChanged, OnCheckedChanged, OnClick)
+- **TC25** — Combined with DataBind pattern (event wiring + data binding on same page)
+
+## See Also
+
+- [Event Handler Signatures](Phase2-EventHandlerSignatures.md) — Code-behind signature transforms
+- [DataBind Pattern Conversion](Phase3-DataBindConversion.md) — DataSource/DataBind → Items binding
+- [Automated Migration Guide](AutomatedMigration.md) — Full list of automated transforms