feat(site): adopt the noctcore theme with the nocturne preset and redesign rule pages

[Shironex]

Summary

The docs site moves onto the noctcore theme's token system with the Nocturne preset (the current night-sky look, refined), and the rule pages, the rule index and the package cards get a redesign. The showcase-kit site uses the sibling Observatory preset from the same token contract (noctcore/showcase-kit#8).

Merge #51 first: this branch contains its bun 1.4.2 commit, so this diff shrinks to the site once that lands.

• Theme (site/src/styles/noctcore/): base.css maps the --nc-* tokens onto Starlight's variables once, components.css holds the shared pieces, and presets/nocturne.css sets the tokens for dark and light and imports its own fonts (Space Grotesk and JetBrains Mono, already self-hosted). site.css keeps this site's own rules. Expressive Code gets Nocturne's code colours, and the existing verdict plugin stays.
• Titles: the site title reads noctcore / ESLint plugins, and a rule's title splits into a quiet namespace (noctcore-contracts/) over the rule name.
• Rule pages: the doc's blockquote becomes the lead paragraph. The old metadata line becomes a facts strip, the same cells in the same place on every page:
  • an ESLint rule: package and version, recommended preset, autofix, suggestions, options, type information;
  • a lint-meta rule: its factory, entry point and category.
    The Incorrect and Correct frames get a tinted title bar, a coloured edge and a cross or tick, using the existing verdict-* classes.
• All rules and the package tables: summary numbers over the index and a heading row per package. Text badges replace the emoji, with opt-in and "needs options" markers. Below phone width, rows stack into labelled entries. rules.json and llms.txt keep their shape.
• Package cards: short name, version, and a bar for the rules on, off and left out of the preset.
• Landing: "Yes, if" and "Probably not, if" side by side, with layout wrappers only and no prose change.
• Telemetry: every astro script sets ASTRO_TELEMETRY_DISABLED=1, because Astro only skips its usage telemetry on CI.
• Tests: a token contract test (the preset sets every token the theme reads, in dark and light) and a facts-strip test.
• CONTRIBUTING: now describes the facts strip.

A fix worth knowing: Starlight's Markdown content puts a top margin between sibling elements, which broke the grid pieces. Their cells reset it in components.css.

Test plan

• bun install --frozen-lockfile, bun run build, bun run typecheck, bun run test (ESLint 10 and 9)
• bun run docs:build: 130 pages, 113 rule pages for 113 docs, 18575 root-relative links, rules.json 113 entries with 113 URLs
• bun run --cwd site test (40 pass); bun run docs:readmes leaves the tree unchanged
• 5 mutation checks went red: a token removed from the preset in dark and in light (contract test), the opt-in marker on a deprecated rule, a negative facts cell without its quiet style, and the blockquote emitted instead of the lead
• Contrast on the built pages (computed colours, WCAG formula): lowest text pair 5.64:1 dark and 4.97:1 light; frame edges and focus 3:1 or better; no horizontal scroll at 390 px
• Screenshots of the splash, getting started, a rule page, the all-rules index and a package page in dark and light at 1440 and 390 px
• After merge: the deployed site renders the same