feat(text): expose trimmedHeight on text nodes

[chiefcll]

Summary

Adds a read-only node.trimmedHeight getter and trimmedHeight to the text loaded event payload. Represents the visible glyph extent — first line's cap-top to last line's descender bottom — independent of lineHeightPx's half-leading and ascender slack.

The intended use case is flex layouts:

text.once('loaded', (n, payload) => {
  text.h = payload.trimmedHeight;
});

After this, a flex parent with alignItems: 'center' will optically center the caps rather than the full lineHeightPx × N line-box. No more "the text looks a few pixels low" inside flex rows.

API

• CoreTextNode#trimmedHeight: number — read-only getter (inherited automatically by ITextNode).
• NodeTextLoadedPayload.trimmedHeight: number — new field alongside dimensions.
• TextRenderInfo.trimmedHeight: number — new field (also on the cached TextLayout).

Formula

For N lines (descender is negative in NormalizedFontMetrics, so subtracting it adds the depth):

trimmedHeight = capHeight − descender + (N − 1) × lineHeightPx

• 1 line: capHeight − descender (cap-top to descender bottom).
• Empty text: 0.
• Interior lines retain the full lineHeightPx between baselines — only the first cap-top and last descender are trimmed.

Matches CSS text-box-trim: trim-both; text-box-edge: cap text.

Decisions

Asked the user and locked:

• Edges: cap-top → descender (rejected: cap-top → baseline; ascender → descender; per-node configurable).
• API shape: read-only getter + loaded event payload (rejected: auto-apply trimToContent prop, both).

Files

• src/common/CommonTypes.ts — NodeTextLoadedPayload.trimmedHeight.
• src/core/text-rendering/TextRenderer.ts — TextRenderInfo.trimmedHeight, TextLayout.trimmedHeight.
• src/core/text-rendering/SdfTextRenderer.ts — compute once in generateTextLayout, cache; populate in all return paths.
• src/core/text-rendering/CanvasTextRenderer.ts — same formula in renderText.
• src/core/CoreTextNode.ts — initialize on _renderInfo, emit in loaded event, expose getter.

Test plan

• pnpm build clean.
• vitest run — 182/182 pass.
• VRT recert not expected (no rendering behavior change), but worth running once before merge.

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