feat(text): optical-center baseline mode, default for mixed-case UI

[chiefcll]

Summary

Pure cap-height centering (PR #27's default) reads slightly low for lowercase-heavy mixed-case strings like "Button" — the visual mass sits in the x-height band, below cap-center. Pure x-height centering goes the other way: capitals appear high.

This PR adds a new textBaselineMode: 'optical' that places the mean of cap-height and x-height at the line-box center:

firstBaselineY = (lineHeightPx + (capHeight + xHeight) * 0.5) * 0.5

This is the sweet spot for typical TV UI text — the same heuristic macOS/iOS controls use. 'optical' becomes the new default. 'cap', 'x', and 'linebox' remain available for content where one of those is specifically preferred.

Why

For Ubuntu at common UI sizes, optical-mode shifts text up vs. cap-mode by:

fontSize	shift up
32 (button label)	~1.7 px
44 (heading)	~2.3 px
50 (large heading)	~2.75 px

That's the "text reads a little low" nudge designers manually apply in CSS — now built into the engine.

When to override

• All-caps or numeric content (timers, badges, all-uppercase headings) → set textBaselineMode: 'cap' globally. Optical mode pushes caps slightly above center for these.
• Body text where lowercase dominates → 'x' if you want maximum lowercase centering.
• Legacy line-box layouts → 'linebox' matches the pre-PR-feat(text): cap-height centering as the per-line vertical anchor #27 behavior.

Files

• src/core/text-rendering/TextRenderer.ts — extended TextBaselineMode type and docstring.
• src/core/text-rendering/TextLayoutEngine.ts — added 'optical' branch and updated the long anchor comment; switched module-level default.
• src/main-api/Renderer.ts — updated RendererMainSettings.textBaselineMode docstring and ?? 'optical' fallback.
• src/core/text-rendering/SdfFontHandler.ts — corrected stale comment: xHeight is now also consumed by 'optical', not just 'x'.
• 142 VRT snapshots in visual-regression/certified-snapshots/chromium-ci/ recertified to reflect the new default.

Test plan

• pnpm build clean.
• vitest run — 182/182 pass.
• Full VRT in Docker CI mode — 166/166 pass against the new certified set.
• Visually spot-checked text-vertical-align snapshots — baselines now sit slightly above the green cap-mode reference lines, exactly the expected ~2.75 px optical shift at fontSize: 50.

Notes for reviewers

• This is a default-changing PR. Apps that depend on the exact pre-PR baseline positions (pixel-perfect manual layouts pinned to cap-anchored baselines) will see a small upward shift on every text node. The fix is one line: textBaselineMode: 'cap' in renderer settings.
• The trimmed-height formula from feat(text): expose trimmedHeight on text nodes #30 is unchanged — it's still measured from cap-top to descender bottom regardless of baseline mode.

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