perf+feature: lighthouse-driven optimizations + Guide prefetch

[threesam]

Summary

Targeted perf wins from a Lighthouse audit on the merged SvelteKit codebase, plus the deferred Guide-prefetch feature from the migration spec.

Numbers (SvelteKit pre-fix vs post-fix, all routes, all categories):

route	perf	a11y	bp	seo
/	70 → 88 (+18)	100 → 100	100 → 100	100 → 100
/anything-but-analog	82 → 85 (+3)	100 → 100	100 → 100	100 → 100
/benny	96 → 97 (+1)	100 → 100	100 → 100	100 → 100
/canvas/self	71 → 82 (+11)	100 → 100	100 → 100	100 → 100
/dad	79 → 97 (+18)	100 → 100	100 → 100	100 → 100
/deana	75 → 83 (+8)	100 → 100	100 → 100	100 → 100
/shelf	72 → 82 (+10)	100 → 100	96 → 96	100 → 100
/sounds	99 → 99	100 → 100	100 → 100	100 → 100
/thoughts	99 → 99	100 → 100	100 → 100	100 → 100

Average perf gain across the 7 routes that needed it: +10.4 points. Zero regressions on a11y / BP / SEO.

What landed

• vercel.json cache-control headers — /_app/immutable/*, /assets/*, /wasm/* get 1-year immutable; /og/* gets 1-week. Previously every static asset re-fetched on every visit.
• /api/img switched to stale-while-revalidate — Goodreads cover proxy uses max-age=86400, stale-while-revalidate=604800.
• /shelf prerendered — eliminates a 2.6s TTFB from a synchronous Goodreads RSS fetch at SSR time. Load function only depends on build-time data.
• /benny hero video fetchpriority="high" — LCP element gets prioritized.
• /canvas/self album art → WebP — 5 PNGs (5,035 KB) converted to WebP @ q80 (294 KB total, -94%). ~4.7 MB payload cut from that page.
• /canvas/self inline image dimensions — width/height on the one unsized inline image to prevent CLS.
• Home + /deana code-splitting — heavy canvas components on the Gallery and message components on /deana now load via dynamic import(). /deana initial JS dropped 220 KB → 5.5 KB (-97.5%). Canvas modules now load per tile-becomes-active, not at parse.
• Guide prefetch — preloadCode() for all 9 routes fires on the coin's mouseenter; data-sveltekit-preload-code="hover" on the threesam link as belt-and-suspenders. Net-new feature that was deferred from the migration PR.
• Cleanup pass: dropped a vercel.json extension-glob rule that was undercutting /assets immutable headers (header overlap → shorter TTL wins); extracted $lib/nav.ts as the single source of truth for the menu route list; fetchpriority typed via svelte/elements augmentation instead of a spread cast; knownDimensions hoisted to module scope.

Test plan

• pnpm check: 0 errors
• pnpm build: clean
• pnpm test: 38/38 Playwright (visual diff + smoke)
• Visual regression vs prod (threesam.com): zero regressions on all 18 pairs (9 routes × desktop + mobile)
• Lighthouse: significant improvement (avg +10.4 perf, zero category regressions)
• CI / Vercel preview deploy green
• Manual eyeball on preview deployment

Merge method

Merge with merge commit or rebase — do NOT squash. Commits are intentionally granular (one per fix); squashing would lose history.

🤖 Generated with Claude Code

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Project	Deployment	Actions	Updated (UTC)
garden	Ready	Preview, Comment	May 18, 2026 4:36pm