fix(text): baseline-anchored layout, include lineGap, trim trailing letterSpacing

[chiefcll]

Summary

Aligns SDF and Canvas text layout with CSS line-box semantics. Multiple fonts at the same size now share a baseline, default line spacing matches the web's normal line-height, and trailing letter-spacing no longer skews alignment.

Changes

TextLayoutEngine.mapTextLayout

• bareLineHeight now includes lineGap (CSS normal = asc + lineGap - desc).
• line[4] is the alphabetic baseline Y in screen px, not the line-box top.
• Renamed halfDelta → halfLeading; computed firstBaselineY = halfLeading + ascender.
• After lines are wrapped/measured, one trailing letterSpacing is trimmed from each non-empty line's reported width so effectiveMaxWidth and textAlign center/right offsets no longer over-count by one letter-spacing. Wrap decisions still use the un-trimmed accumulators (no wrap-behavior regressions).

SdfTextRenderer

• Anchors glyphs against the alphabetic baseline using common.base: glyph.y = baselineY + yoffset − atlasBase. Fonts whose BMFont base differs (e.g. Ubuntu 32.6 vs Noto 44.9 at design size 42) now sit on the same line at the same fontSize.
• Dropped the SDF-specific designLineHeight fallback so the engine's lineHeight × bareLineHeight (incl. lineGap) is used uniformly with Canvas.
• fontStyle added to the layout cache key.

CanvasTextRenderer

• Both measuring and drawing contexts switched from textBaseline = 'hanging' to 'alphabetic', matching the new line[4] semantics.

Why

Previous code:

1. Dropped lineGap from default line height (~6% tighter than the browser for Ubuntu).
2. Treated BMFont's line-box top as the engine's line-box top, so two fonts with different common.base could not share an on-screen baseline.
3. Added trailing letterSpacing to each line width, shifting center/right alignment by letterSpacing px.
4. Canvas/SDF used different baseline conventions ('hanging' vs ad-hoc yoffset placement), so the two backends could not produce identical layouts for the same input.

Reviewer notes

• Expected visual-regression diffs: default-line-height text is taller (lineGap added). Multi-font scenes share a baseline. Centered/right-aligned text with non-zero letterSpacing shifts by ~½ letterSpacing toward the correct CSS position. pnpm test:visual:update will need to be re-run and certified snapshots reviewed.
• verticalAlign is not implemented in this PR — it's the next change. A handful of related items (e.g. gating maxHeight truncation on contain) are intentionally deferred to land with that work.
• The lineHeight <= 3 unitless-multiplier heuristic is kept as-is (API contract).

Test plan

• pnpm test --run (148/148 pass)
• pnpm build clean
• Prettier/eslint clean on changed files
• pnpm test:visual:update and review certified snapshot diffs before merge

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