fix(text): bold and italic rich text runs no longer overlap following text

[wouterlucas]

Part 2 of 4. Base is #907 — review that first; this diff is isolated against it.

Fixes the reported defect: with richText: true, The [b]brown[/b] fox renders as brownfox.

Root cause

Styled spans were drawn with a styled face but advanced by unstyled metrics.

Canvas. The draw context was switched to the span font (CanvasTextRenderer.ts:286-289), but the advance came from CanvasFontHandler.measureText, which ignores its fontFamily argument and measures against a shared context still set to the base font (CanvasFontHandler.ts:226-243, primed at CanvasTextRenderer.ts:146). A bold segment therefore advanced by its regular-face width. The deficit — typically 3-6% of segment width — consumes the following space.

SDF. Bold is faked by shifting the shader alpha threshold (SdfShader.ts:178, threshold = 0.5 - v_style * 0.05) and italic by shearing (ITALIC_SHEAR), neither of which touches xadvance (SdfTextRenderer.ts:618). Milder than Canvas — crowding rather than overlap — but wrong.

Changes

• setMeasureFont / getMeasureFont on CanvasFontHandler so the measure context tracks the draw context. Restored to the base font afterwards, since the context is shared with calculateFontMetrics and the plain text path.
• SDF bold adds the width the threshold shift actually produces, 2 * shift * distanceRange, centred on the advance by shifting x1 by half.
• An italic run pays its shear overhang when the run ends, tracked lazily so no lookahead is needed.
• Kerning suppressed across a style boundary, where the pair adjustment from the regular face does not apply.
• Shared formulas in a new RichTextMetrics module so measurement and drawing cannot drift apart. The SDF bold constant sits next to a note pointing at the matching shader threshold — changing one without the other reintroduces the bug.

The width correction is not optional

effectiveMaxWidth (TextLayoutEngine.ts:110-121) is the longest base-font-measured line, and in CanvasTextRenderer.ts it both sizes the texture (:176, :192) and is published as the node width (:408).

Fixing the advance alone would push drawn glyphs past canvasW, trading overlap for right-edge clipping, and still under-report width to siblings. So both renderers re-measure each final line with span fonts and recompute effectiveWidth before the canvas is sized. Alignment offsets are recomputed alongside, since non-left alignments reference the widest line.

Known limitation

Line breaking is still base-font based, so a styled line can wrap slightly early or late. That needs style-aware measurement inside the layout engine — PR 3 of this stack.

Testing

tsc --noEmit clean, 325 tests pass, eslint 0 errors. Adds 20 tests: RichTextMetrics unit coverage, plus a CanvasFontHandler suite using a context stub whose measureText depends on the active font, pinning that a styled measurement of The [b]brown[/b] fox exceeds the base measurement.