feat: resolve rem/em font units to px before canvas measurement

[okaris]

Problem

Canvas ctx.font silently ignores relative CSS units (rem, em). When callers pass CSS-derived font strings like "0.9rem Inter", canvas keeps its previous font and returns wrong widths. This is a spec limitation — canvas can exist without a document root, so relative units have no reference point (WHATWG HTML #1682).

In practice, anyone using Tailwind or a CSS framework with custom text-sm sizes (e.g. --text-sm: 0.9rem instead of the default 0.875rem) gets silently wrong measurements. The text measures at whatever font was previously set on the canvas context, not at the intended size.

Related: #107 (canvas doesn't account for certain CSS properties).

Solution

Add resolveFont() in measurement.ts that rewrites relative units to absolute px before the string reaches canvas:

"0.9rem Inter" → "14.4px Inter"  (given 16px root)
"bold 1.5em mono" → "bold 24px mono"
"14px Inter" → "14px Inter"  (no-op, no allocation)

Resolution uses getComputedStyle(document.documentElement).fontSize — the same level of DOM access that getEmojiCorrection() already does. Results are cached at module level so the DOM read happens once. Environments without a document (OffscreenCanvas, Node.js) fall back to 16px.

Changes

• resolveFont(font) — exported, cached, handles rem/em → px
• getFontMeasurementState() — resolves font before ctx.font = and cache keying
• parseFontSize() — resolves before extracting the px value
• clearMeasurementCaches() — clears the resolution cache + cached root font size
• Tests for px passthrough, rem resolution, em resolution, and parseFontSize integration

Test plan

• bun test src/layout.test.ts — 85 tests pass (5 new)
• Existing tests unaffected (all use px fonts, passthrough is zero-cost)
• No document in test env → falls back to 16px root, rem math still correct