Add MultilineEditControl documentation, update rendering pipeline docs

MultilineEditControl.md: comprehensive documentation covering all features
including syntax highlighting (ISyntaxHighlighter), pluggable gutter system
(IGutterRenderer), find/replace API, undo/redo, word wrap modes, keyboard
shortcuts, mouse interaction, per-line highlights, and extensibility examples
(breakpoint gutter, code editor, log viewer patterns).

CONTROLS.md: linked MultilineEditControl with updated description.

RENDERING_PIPELINE.md: updated SetCell documentation to reflect the
SetCell(Cell) / SetNarrowCell(char/Rune) API split. The old
SetCell(x, y, Rune, Color, Color) was renamed to SetNarrowCell to
make misuse a compile error.

Author: nickprotop

docs/CONTROLS.md

@@ -37,7 +37,7 @@ Controls for user input and interaction.
 | **[DatePickerControl](controls/DatePickerControl.md)** | Locale-aware date picker | Segmented editing, calendar popup, min/max dates |
 | **[PromptControl](controls/PromptControl.md)** | Single-line text input | Enter key events, input validation, max length |
 | **[TimePickerControl](controls/TimePickerControl.md)** | Locale-aware time picker | 12h/24h modes, seconds toggle, min/max times |
-| **MultilineEditControl** | Multi-line text editor | Scrolling, word wrap, text selection |
+| **[MultilineEditControl](controls/MultilineEditControl.md)** | Multi-line text editor | Syntax highlighting, pluggable gutter, find/replace, undo/redo, word wrap |
 | **[SliderControl](controls/SliderControl.md)** | Value slider with thumb | Horizontal/vertical, step/large-step, keyboard/mouse drag |
 | **[RangeSliderControl](controls/RangeSliderControl.md)** | Dual-thumb range slider | MinRange enforcement, tab to switch thumbs, range events |
 

docs/RENDERING_PIPELINE.md

@@ -447,14 +447,26 @@ private void PaintStage(CharacterBuffer buffer)
 
 **CharacterBuffer key methods:**
 ```csharp
-// Individual cell (char overload wraps in Rune internally)
-void SetCell(int x, int y, Rune character, Color fg, Color bg);
+// Full Cell (preserves flags: IsWideContinuation, Combiners, Decorations)
+// Use for parsed markup cells and cell copies between buffers
+void SetCell(int x, int y, Cell cell);
+
+// Narrow character (clears all flags, assumes width-1)
+// Use for literal characters: borders, padding, fill chars, track chars
+void SetNarrowCell(int x, int y, char character, Color fg, Color bg);
+void SetNarrowCell(int x, int y, Rune character, Color fg, Color bg);
+
 // Text rendering with automatic wide character and combiner handling
 void WriteString(int x, int y, string text, Color fg, Color bg);
+
 // Bulk fill
 void FillRect(LayoutRect rect, char c, Color fg, Color bg);
 ```
 
+> **Note:** The old `SetCell(x, y, Rune, Color, Color)` overload was renamed to `SetNarrowCell`
+> to make misuse a compile error. Use `SetCell(Cell)` for parsed/copied cells, `SetNarrowCell`
+> for literal narrow characters.
+
 #### Stage 3.5: Pre/Post Buffer Paint Hooks (Compositor Effects)
 *File: `Windows/WindowRenderer.cs`*
 
@@ -578,13 +590,18 @@ R1 │ 'l', W, B │   │ 'o', W, B │   │ ' ', W, B │
 
 ### Key Operations
 
-#### 1. SetCell (Individual Cell Update)
+#### 1. SetCell / SetNarrowCell (Individual Cell Update)
 *File: `CharacterBuffer.cs`*
 
-Accepts both `char` (convenience overload, wraps in `Rune`) and `Rune` (primary):
+Two write APIs with distinct purposes:
+
+- **`SetCell(int x, int y, Cell cell)`** — preserves all flags (`IsWideContinuation`, `Combiners`, `Decorations`). Use for cells from `MarkupParser.Parse()` or when copying cells between buffers.
+- **`SetNarrowCell(int x, int y, char/Rune, Color fg, Color bg)`** — clears all flags, assumes width-1. Use for literal narrow characters: border chars, padding spaces, fill chars.
+
+> The old `SetCell(x, y, Rune, Color, Color)` overload was renamed to `SetNarrowCell` to make misuse a compile error.
 
 ```csharp
-public void SetCell(int x, int y, Rune character, Color foreground, Color background)
+public void SetNarrowCell(int x, int y, Rune character, Color foreground, Color background)
 {
     if (x < 0 || x >= Width || y < 0 || y >= Height)
         return;