[arzafran]

What this does

Fonts get checked at every performance audit instead of only when someone remembers to paste the subsetting prompt at project handoff. /audit performance now measures each self-hosted font and what the site actually uses. A font carrying characters, weights or features the site never uses, or one that's preloaded where nothing above the fold needs it, becomes a measured finding, and the subsetting procedure from #273 is the fix.

Linear: OSS-127

Summary

• skills/audit/SKILL.md, Performance mode:
  • Phase 0: a web app that serves its own font files always gets a font surface in the measurement plan.
  • Phase 1 step 4 (new): per font file, bytes, glyphs, axes, OpenType features and hinting; per key route, font bytes and preloads. Compared against what the site uses, per step 2 of "Subsetting fonts without changing rendering" in docs/performance-reference.md. The finding's direction is that procedure, so pixel diffs and a person's sign-off on any visible change come with it. A repo with a subset script gets its --check run, and a missing CI step for it is a LOW finding.
• src/lib/lint-skills.ts: the audit skill's line limit drops from 696 to 694, because the change made the file shorter.
• evals/audit-perf-fonts: a Next app that can't be built, with a preloaded variable font, weights 400 and 650, ss01, accented characters and hyphens: auto. The audit has to plan the font measurements, tie them to what the site uses, and report no font numbers it couldn't measure.

Test Plan

• bun run typecheck exits 0
• bun run test: 3144 pass, 0 fail
• bun run lint:skills: 0 errors
• All 5 audit eval cases pass, including the new one ($1.24)
• Next real perf audit on a client site reports its fonts