Add calculator plugin (interactive `calc forms in Markdown)

A new first-party plugin that turns a fenced `calc` block written in
YAML into a client-side calculator: labelled inputs (number/range/select)
plus outputs computed live from expressions referencing the input names.

- Preprocessor (order 15) replaces each calc fence with a raw-HTML
  <form class="nd-calc"> that Markdig passes through.
- A single vanilla-JS evaluator is injected once per page; it builds a
  Function from each sanitised expression with the inputs as named
  variables (JS Math is in scope) and recomputes on input. Re-binds on
  Material's document$ so it survives instant navigation.
- Expressions are restricted to a safe character set (no quotes/semicolons/
  brackets/backslashes); unsafe outputs are dropped with a warning.
- Lightweight number formatting (e.g. ".00", "0.0 kWh").
- Theme CSS for the input grid and result cards (palette-aware).

Registered as "calculator" (alias "calc"), enabled in the dogfood site
with a live power-cost example. Adds plugins/calculator.md + nav entry and
10 unit tests (198 total).

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

Author: Netdocs

docs-site/appsettings.json

@@ -70,6 +70,7 @@
               { "path": "plugins/arithmatex.md" },
               { "path": "plugins/b64.md" },
               { "path": "plugins/blog.md" },
+              { "path": "plugins/calculator.md" },
               { "path": "plugins/file-filter.md" },
               { "path": "plugins/git-revision-date.md" },
               { "path": "plugins/glightbox.md" },
@@ -119,6 +120,7 @@
       { "name": "tags", "options": { "export": true } },
       { "name": "meta" },
       { "name": "arithmatex" },
+      { "name": "calculator" },
       { "name": "social" }
     ],
 

docs-site/docs/plugins/calculator.md

@@ -0,0 +1,130 @@
+---
+title: calculator
+---
+
+# calculator
+
+Author an **interactive calculator** entirely in Markdown. A fenced <code>```calc</code>
+block written in YAML becomes a small client-side form: labelled inputs plus outputs that
+recompute live as you type. No server, no build-time math — just a tiny vanilla-JS evaluator.
+
+It is a first-party Netdocs plugin (there is no MkDocs equivalent). Enable it in `plugins`:
+
+```json title="appsettings.json"
+{ "name": "calculator" }
+```
+
+## Example
+
+````markdown
+```calc
+title: Power cost
+inputs:
+  - name: watts
+    label: Power draw (W)
+    default: 100
+  - name: hours
+    label: Hours per day
+    default: 24
+  - name: rate
+    label: Price ($/kWh)
+    default: 0.12
+    step: 0.01
+outputs:
+  - label: Daily cost
+    expr: (watts * hours / 1000) * rate
+    format: "$0.00"
+  - label: Monthly cost
+    expr: (watts * hours / 1000) * rate * 30
+    format: "$0.00"
+```
+````
+
+Renders as:
+
+```calc
+title: Power cost
+inputs:
+  - name: watts
+    label: Power draw (W)
+    default: 100
+  - name: hours
+    label: Hours per day
+    default: 24
+  - name: rate
+    label: Price ($/kWh)
+    default: 0.12
+    step: 0.01
+outputs:
+  - label: Daily cost
+    expr: (watts * hours / 1000) * rate
+    format: "$0.00"
+  - label: Monthly cost
+    expr: (watts * hours / 1000) * rate * 30
+    format: "$0.00"
+```
+
+## How it works
+
+The plugin runs as a Markdown preprocessor (order `15`) and replaces each `calc` fence with
+a raw-HTML `<form class="nd-calc">`. A single evaluator script is injected **once per page**;
+it collects each form's inputs into named variables, evaluates every output expression, and
+updates the results on every `input`/`change`. It also re-binds on Material's `document$`
+observable, so calculators keep working with instant navigation.
+
+## Inputs
+
+Each entry under `inputs`:
+
+| Field | Type | Required | Description |
+|---|---|---|---|
+| `name` | string | yes | Variable name used in expressions. Must be a valid identifier (`[A-Za-z_][A-Za-z0-9_]*`). |
+| `label` | string | no | Field label (defaults to `name`). |
+| `type` | string | no | `number` (default), `range`, or `select`. Providing `options` implies `select`. |
+| `default` | number/string | no | Initial value. |
+| `min` / `max` / `step` | number | no | Passed through to the `<input>` (useful for `number`/`range`). |
+| `options` | array | no | For selects — a list of strings or `{ value, label }` objects. |
+
+A `range` input also renders a small live readout of its current value.
+
+## Outputs
+
+Each entry under `outputs`:
+
+| Field | Type | Required | Description |
+|---|---|---|---|
+| `label` | string | no | Result label (defaults to `Result`). |
+| `expr` | string | yes | Expression evaluated with the input names as variables. |
+| `format` | string | no | Number format (see below). |
+
+### Expressions
+
+Expressions are evaluated in the browser with the inputs available as variables and the
+JavaScript [`Math`](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Math)
+object in scope, so `Math.round`, `Math.min`, `Math.pow`, ternaries (`a > b ? x : y`), etc.
+all work:
+
+```yaml
+outputs:
+  - label: Rounded
+    expr: Math.round(watts * hours / 1000)
+```
+
+!!! warning "Sanitised input"
+    Expressions are restricted to a safe character set (digits, identifiers, arithmetic and
+    comparison operators, parentheses, commas). Quotes, semicolons, brackets and backslashes
+    are rejected, and such an output is dropped with a build warning — so a stray expression
+    can't inject arbitrary script into the page.
+
+### Formats
+
+`format` is a lightweight pattern: the first `0` / `0.0…` token sets the number of decimal
+places, and any surrounding text is kept as a prefix/suffix.
+
+| `format` | `1234.5` renders as |
+|---|---|
+| *(omitted)* | `1234.5` |
+| `0` | `1235` |
+| `0.00` | `1234.50` |
+| `$0.00` | `$1234.50` |
+| `0.0 kWh` | `1234.5 kWh` |

src/Netdocs.Cli/CliApp.cs

@@ -233,6 +233,7 @@ private static async Task<int> ImportAsync(string[] args)
         .Register<LinkNotesPlugin>("link-notes", "affiliate-links")
         .Register<Base64EmbedPlugin>("b64", "pymdownx.b64")
         .Register<ArithmatexPlugin>("arithmatex", "pymdownx.arithmatex")
+        .Register<CalculatorPlugin>("calculator", "calc")
         .Register<MacrosPlugin>("macros");
 
     private static string? ResolveConfigPath(string? provided)