fix(fonts): match Ghostty CJK width scaling

[wiedymi]

Summary

• replace the wide-fallback primary-em shortcut with Ghostty's default em-normalized ic_width adjustment
• measure the primary printable-ASCII box and the fallback 水 advance, with Ghostty-compatible ex-height, cap-height, and line-height fallbacks
• share the metric implementation between WebGPU and WebGL2 while preserving the common baseline and two-cell safety cap
• add regression coverage using the bundled JetBrains Mono and Noto Sans CJK metrics
• prepare restty 0.2.4

Root cause

Version 0.2.3 forced wide fallback faces to the primary em scale but bypassed the existing metric adjustment. Ghostty instead scales ordinary fallback faces by an em-normalized ic_width ratio. For the bundled fonts, this ratio is 1050 / 1000 = 1.05; retaining it increases the CJK advance from 13.64 px to 14.32 px in a 16 px two-cell span and removes the excess whitespace.

Validation

• bun test tests/fallback-font-layout.test.ts tests/symbol-rendering.test.ts
• PowerShell equivalent of bun run test:ci: 316 tests passed
• live bundled-font metric probe: metricAdjust=1.05, advancePx=14.3181818
• bun run build:types
• bun run check:themes
• bun run format:check
• bun run lint
• bun run build
• bun run playground:build
• release-note extraction for 0.2.4

Fixes #24

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