Add OpenGraph/Twitter cards, generated per sin at build time

Link previews are the funnel for a "send the link" site, so give every sin
a share-optimized card.

- og/twitter meta tags on every page (poke as description, canonical URL,
  summary_large_image)
- Per-sin 1200x630 card generated at build: the poke, the before/after
  charts (uniform height so tall vconcat charts fit), severity meter,
  category, and short citations — composed with satori, rasterized with
  @resvg/resvg-js, no browser needed
- Default brand card for the homepage and canon
- Shared chart pipeline extracted to lib/renderChart.ts (used by both the
  on-page component and the card generator)
- Bundle three IBM Plex TTF weights for the rasterizer

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XPEvmMbac2fCvKXpQcovj8

src/pages/og/[slug].png.ts

@@ -0,0 +1,24 @@
+import type { APIRoute, GetStaticPaths } from 'astro';
+import { getCollection } from 'astro:content';
+import { getChartSpec } from '../../lib/charts';
+import { renderSinCard } from '../../lib/og';
+
+export const getStaticPaths: GetStaticPaths = async () => {
+  const sins = await getCollection('sins', ({ data }) => !data.draft);
+  return sins.map((sin) => ({ params: { slug: sin.id }, props: { sin } }));
+};
+
+export const GET: APIRoute = async ({ props }) => {
+  const { sin } = props as { sin: Awaited<ReturnType<typeof getCollection>>[number] };
+  const png = await renderSinCard({
+    poke: sin.data.poke,
+    category: sin.data.category,
+    severity: sin.data.severity,
+    badSpec: getChartSpec(sin.data.badChart),
+    fixedSpec: getChartSpec(sin.data.fixedChart),
+    citationKeys: sin.data.citations.map((c) => c.key),
+  });
+  return new Response(new Uint8Array(png), {
+    headers: { 'Content-Type': 'image/png', 'Cache-Control': 'public, max-age=31536000, immutable' },
+  });
+};

src/pages/og/default.png.ts

@@ -0,0 +1,9 @@
+import type { APIRoute } from 'astro';
+import { renderDefaultCard } from '../../lib/og';
+
+export const GET: APIRoute = async () => {
+  const png = await renderDefaultCard();
+  return new Response(new Uint8Array(png), {
+    headers: { 'Content-Type': 'image/png', 'Cache-Control': 'public, max-age=31536000, immutable' },
+  });
+};