feat(canvas): measure labels with real font metrics via engine callback (DasherCore v0.2.4)

[willwade]

Summary

Wires DasherCore v0.2.4's dasher_set_text_size_callback (#28's Windows-side actions; engine fix = DasherCore #56 / PR #57): the engine now asks the frontend to measure single-line labels with real font metrics instead of its code-points × fontSize/2 estimate. The estimate under-measured wide Segoe UI glyphs by up to ~2× and the error compounded down the ancestor label chain — long sentences ended with jumbled, squashed, cut-off labels at the right edge (the reporting user's screenshots; she runs v5 on Windows without the problem).

• measuring and drawing share CommandRenderer.ResolveTypeface (user-selected Dasher font, or Segoe UI) so layout can never disagree with rendering
• dasher_text_metrics_changed() invalidates the engine's measurement cache whenever SP_DASHER_FONT changes (font picker, settings import)
• measurement failures return 0 → the engine falls back to its estimate without caching and retries next frame
• callback fires on the dasher_frame() thread = the UI thread (DispatcherTimer), so FormattedText is safe

Submodule pin: v0.2.4 (bumped in the stacked #32).

Issue / RFC: #28, DasherCore #56, DasherCore #57

Type of change

• Bug fix (v5-parity rendering regression)
• New feature
• Cross-platform / parity change (Windows first; GTK/Android/web can adopt the same callback — engine seam is documented in DasherCore v0.2.4 release notes)

Cross-platform impact

• This changes a capability that users see on other platforms. (frontend-local fix; other frontends get the same fix by wiring the same seam)
• This introduces a new UX or hardware interaction.

Manual verification (RFC 0011)

• Build green, 0 warnings; suite 43/43
• Smoke run against v0.2.4 DLL: 12s session, canvas renders, no faults
• Acceptance test = the user's screenshots: long-sentence run shows frontier labels staying legible at the right edge; re-run after changing the canvas font

Definition of Done

• CI is green
• Tests added for new behaviour (measurement needs Avalonia's text pipeline — manual pass per RFC 0011; engine caches per label × size so steady-state makes zero callbacks)
• Feature matrix updated (n/a — correctness fix)
• Docs updated (DasherCore v0.2.4 release notes document the seam)
• Commits are signed off (DCO)

Stacked on #30 → #31 → #32. Fixes #28

Greptile Summary

The PR integrates DasherCore’s text-size callback so label layout uses the same Avalonia typeface metrics as rendering.

• Registers and roots the native text-measurement callback.
• Refreshes the selected font and invalidates cached metrics before each affected frame.
• Centralizes typeface resolution between measurement and drawing.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
src/Dasher.Windows/Controls/DasherCanvas.cs	Registers the text-size callback, measures labels through Avalonia, and refreshes and invalidates font metrics before frame generation; both previously reported lifecycle and ordering issues are fixed.
src/Dasher.Windows/Engine/CommandRenderer.cs	Extracts shared typeface resolution so measurement and rendering select the same font.
src/Dasher.Windows/Engine/NativeBridge.cs	Adds the DasherCore v0.2.4 callback delegate, registration export, and metric-cache invalidation export.

Sequence Diagram

sequenceDiagram
    participant Timer as UI DispatcherTimer
    participant Canvas as DasherCanvas
    participant Core as DasherCore
    participant Renderer as CommandRenderer
    Timer->>Canvas: OnTick()
    Canvas->>Core: Read SP_DASHER_FONT
    alt Font changed
        Canvas->>Core: dasher_text_metrics_changed()
    end
    Canvas->>Core: dasher_frame()
    Core->>Canvas: OnTextSize(label, size)
    Canvas->>Renderer: ResolveTypeface(_dasherFont)
    Canvas-->>Core: measured width and height
    Core-->>Canvas: drawing commands
    Canvas->>Renderer: Render(..., _dasherFont)

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[willwade]

@greptile re-review