feat(#479): clickable inline RichTextHyperlink (OnClick mode)

[codemonkeychris]

Closes #479.

Adds Action? OnClick to RichTextHyperlink and a new factory overload Hyperlink(string text, Action onClick). When OnClick is set the WinUI Hyperlink.Click event fires the delegate and NavigateUri is left unset so the platform does not also navigate. Use to make a single rich-text run interactive (open an editor, dispatch a command) inside a virtualized list of fragment rows — much lighter than InlineUI(Button(...)) because no UIElement is hosted, just a click-wired Documents.Hyperlink.

API shape

RichTextBlock(new[]
{
    Paragraph(
        Run("Set type: "),
        Hyperlink(""string"", () => OpenTypeMenu()),    // ← NEW: clickable inline
        Run("" → ""),
        Hyperlink(""docs"", new Uri(""https://..."")))   // existing navigate mode
})

Reconciler wiring

• Mount: when OnClick is non-null, subscribe a static Click handler and skip NavigateUri.
• Update: handle navigate↔click mode transitions (clear/restore NavigateUri, attach/detach the static handler). The per-Hyperlink action lives in a ConditionalWeakTable<Hyperlink, Action> so closure swaps across renders are a cheap map update — no event detach/attach churn even when callers pass a fresh closure per render.
• CWT keys are weak; entries die when the Hyperlink is GC'd after RichTextBlock.Blocks.Clear() (no leak on full rebuilds).

Drive-by: charting handler registration

While verifying the demo I found that D3Line / D3Path / D3PathTranslated constructed their element records with new() directly, bypassing the spec-048 registration touch in the Line(...) / Path2D() factories. D3Rect and D3Circle already routed correctly; aligned the remaining three so the ControlRegistry entry is installed on first call. Without this any chart that uses lines or paths crashes with ""No handler is registered for element type 'Microsoft.UI.Reactor.Core.LineElement'"" when mounted in a context where Factories.Line(...) / Factories.Path2D() hasn't been touched elsewhere first (e.g. the TestApp's Rich Text demo).

Tests

Tier	What
Unit (MoreCoverageTests)	Factory shape, ArgumentNullException on null action, URI-mode no-op, record-equality tracking OnClick (4 new tests, all green).
Selftest (CoreCov_RichTextHyperlink_OnClickMode)	Click-mode mount has no NavigateUri; identity preserved across closure swap; URI→click transition clears NavigateUri; click→URI restores it. 7 assertions, all green.
Regression	Full Reactor.Tests suite — 9214 passed, 0 failed. All existing RichText/InlineUI selftests still green.

Demo

TestApp's Rich Text + Inline UI page swaps the inline Button re-roll for the new clickable hyperlink so the new shape is exercised end-to-end. Chart paragraphs now mount cleanly thanks to the D3 factory fix.

Docs

docs/_pipeline/templates/text-and-media.md.dt + compiled output updated to list the new Hyperlink(string text, Action onClick) helper.

[Copilot]

Pull request overview

Adds a new “click mode” for rich-text inline hyperlinks so RichTextBlock fragments can be interactive without hosting UIElements, and aligns a few D3 charting helpers with spec-048 handler registration expectations.

Changes:

• Introduces RichTextHyperlink.OnClick plus a new DSL overload Hyperlink(string text, Action onClick) that suppresses navigation and invokes the delegate via Hyperlink.Click.
• Updates the reconciler inline mount/update paths to support navigate↔click transitions with a static click handler + ConditionalWeakTable action storage.
• Fixes D3 charting helpers (D3Line / D3Path / D3PathTranslated) to ensure handler registration is touched before first mount; updates demo + docs; adds unit + selftest coverage.

Show a summary per file

File	Description
tests/Reactor.Tests/MoreCoverageTests.cs	Adds unit tests covering the new Hyperlink(Action) factory and record equality on OnClick.
tests/Reactor.AppTests.Host/SelfTest/SelfTestFixtureRegistry.cs	Registers the new selftest fixture in both the discovery list and constructor switch.
tests/Reactor.AppTests.Host/SelfTest/Fixtures/CoreCoverageFixtures.cs	Adds a selftest verifying click-mode NavigateUri suppression and navigate↔click transitions.
src/Reactor/Elements/Dsl.cs	Adds Hyperlink(string, Action) factory overload with null-checking and sentinel URI.
src/Reactor/Core/Reconciler.Update.cs	Implements click-mode mount/update wiring for RichTextHyperlink using a static handler + ConditionalWeakTable.
src/Reactor/Core/Element.cs	Extends RichTextHyperlink with Action? OnClick and documents the two hyperlink modes.
src/Reactor/Charting/D3Charts.cs	Routes line/path helpers through registration-touch paths (spec-048) to avoid “No handler registered” crashes.
samples/Reactor.TestApp/Demos/RichTextDemo.cs	Updates demo to use the new clickable inline hyperlink instead of InlineUI(Button(...)).
docs/guide/text-and-media.md	Documents the new clickable Hyperlink(string, Action) overload.
docs/_pipeline/templates/text-and-media.md.dt	Updates the docs template source to include the new overload.

Copilot's findings

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• Files reviewed: 10/10 changed files
• Comments generated: 2