feat(core): add fitTextFontSize utility for pixel-accurate text measurement (#152)

Add @chenglou/pretext dependency and fitTextFontSize() utility that uses
canvas measureText to compute the largest font size that fits text within
a given width. Replaces character-count heuristics with actual font-aware
measurement.

- New fitTextFontSize() in @hyperframes/core/text, exposed on window.__hyperframes
- Generalized for all text elements (captions, titles, etc.), not just captions
- Unit tests (mocked pretext) + browser integration test (real Chromium canvas)
- Updated captions skill docs with usage, exit guarantee, and self-lint patterns

Co-authored-by: James <james.russo@heygen.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

skills/hyperframes-compose/SKILL.md

@@ -133,6 +133,7 @@ Video must be `muted playsinline`. Audio is always a separate `<audio>` element:
 - Use `font-display: block` for local fonts — renderer needs fonts loaded before capturing
 - Add `crossorigin="anonymous"` to media loaded from external URLs
 - Minimum readable text: 20px landscape, 18px portrait
+- For dynamic text that may overflow its container, use `window.__hyperframes.fitTextFontSize(text, { maxWidth, fontFamily, fontWeight })` to compute the largest font size that fits on one line. Returns `{ fontSize, fits }`. Options: `baseFontSize` (default 78), `minFontSize` (default 42), `step` (default 2). `fontFamily` and `fontWeight` must match the CSS applied to the element.
 - All files (video, audio, fonts, images) live at the project root alongside `index.html`
 - From sub-compositions, use `../` to reference root files