feat(site): shareable result card + one-tap share to X

[NagaYu]

What & why

Makes a probe result something people want to post, to drive organic spread. Adds a branded, canvas-rendered result card and a one-tap Share on X, plus a static OG image so every shared demo link unfurls a rich preview. Site-only — no src/, public API, report.ts, dist/, or quirks-DB changes; no new runtime deps; no telemetry or network calls.

Changes

• site/share-card.js (new) — renders a ProbeResult to <canvas> at 1200×630 (social) and 1080×1080 (square). System fonts only, offline, dark theme matching --bg/--panel/--accent. Shows only ProbeResult fields (tier badge, hero matmul GFLOPS → shader-compile ms fallback, GPU vendor/arch, browser+OS, wordmark + demo URL). Same privacy stance as src/report.ts — no UA/screen/locale/fingerprinting. Includes an arcTo fallback for engines without ctx.roundRect (pre-2023).
• site/index.html / site/main.js — card preview panel, 1200×630 / 1080×1080 size toggle, and three new buttons in the existing .actions row (reusing current styles): Download card (1200×630 PNG), Copy card image (ClipboardItem, download fallback + hint where unsupported), Share on X (intent/tweet with ≤200-char text + demo URL). renderCard() is wrapped in try/catch so a card failure can never take the probe results down with it. Existing Copy report (Markdown) and Download JSON untouched.
• site/og.png (new) — generic 1200×630 "probe your browser's real WebGPU capability" hero (not a specific result); wired via og:image/og:title/og:description/twitter:card=summary_large_image. Absolute URL matches the live Pages path.
• test/browser/share-card.spec.ts (new) + .github/workflows/ci.yml — Playwright smoke tests (card canvas renders non-blank at correct dims, all 3 buttons present alongside the old actions, size toggle re-renders at 1080×1080, X intent URL carries short text + demo URL). CI browser job now runs prepare-site.mjs first (same step as the Pages deploy) so /site/ has its bundle.

Testing

• pnpm lint, pnpm typecheck, pnpm build, pnpm size all green — core bundle unchanged (4727 B gzip, budget 5120 B); site/ isn't in the library bundle.
• Playwright 15/15 across chromium / firefox / webkit (incl. WASM-fallback engines, which still render the card).
• Verified on a real GPU (Chromium 148 / apple metal-3): webgpu-full, 40.7 GFLOPS card. The roundRect fallback path was exercised by deleting ctx.roundRect at runtime — output is byte-identical to the native path.

Deploy / follow-up (not in this PR)

• Merging to main triggers the Pages deploy (pages.yml) that puts the card + og.png live.
• The X/OG rich preview can only be confirmed after deploy by sharing a real link (X fetches OG server-side and caches). Post to social only after the deploy lands — the current live site has no OG image, so a link shared pre-deploy won't unfurl the card.

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