fix(docs): align guides with generated api

[Exoridus]

Problem

Guides and committed API docs carried pre-existing naming/link drift that PR #94 made highly visible:

• Guides taught a MeshShader class the engine no longer exports — the current API is ShaderSource + MeshMaterial.
• Four prose links pointed at the non-existent /api/mesh-shader/ page.
• core-concepts/application.mdx linked four options interfaces that get no generated API pages.
• SourceSnippet.astro had two pre-existing astro check type errors.
• Running the API generator changed ~12 committed API MDX files (committed output had drifted from source).
• The spine's curated apiLinks were validated but never shown in the chapter UI.

Solution

• MeshShader → MeshMaterial migration. Reworked the canonical custom-mesh-shaders chapter onto the real ShaderSource + MeshMaterial + new Mesh({ material }) API (its construction/attach blocks are now standalone-typechecked instead of no-check), and fixed every prose/table/label reference in custom-mesh-shaders, backend-comparison, custom-renderers, filters, and render-pipeline-debugging (the WebGPU debug label now matches the real MeshMaterial (custom)). Prose now distinguishes shader source / material / renderer correctly; detectUniformDrift() is attributed to ShaderSource.
• API link strategy. Dead /api/mesh-shader/ links now resolve to /api/mesh-material/ (and ShaderSource.detectUniformDrift() → /api/shader-source/#methods). The four *ApplicationOptions / LoaderOptions links are demoted to inline code: the generator documents only classes/enums, so interfaces never get pages, and a four-type generator special case is explicitly out of scope (Option C).
• Astro typecheck cleanup. SourceSnippet.astro derives the language prop from the <Code> component's own lang type (no any, no deep import) and uses Astro.url.pathname for error context instead of the untyped Astro.self.moduleId. astro check is now 0 errors.
• Deterministic API regeneration. Ran the official generator; the ~12-file diff is real source drift (corrected source-line anchors plus new Gradient value-object members and Graphics fill/stroke styles), is byte-identical across runs, and is committed so a fresh build leaves the tree clean.
• Visible spine API links. The chapter layout now renders a compact, keyboard-accessible, focus-visible "Related API" chip row from apiLinks (build-validated against the api collection, only when present); curated TryIt blocks stay for playground + API combos.
• Stronger link validation. New test/site/guide-prose-links.test.ts validates internal /api/ and /guide/ links in MDX prose, NextStep hrefs, and TryIt slugs — with locale / base / trailing-slash / hash / query normalization — and guards against the removed mesh-shader and options-interface pages.

Scope

Docs / site / tooling only. No runtime features, no API changes, no new examples or templates, no playground redesign, no versioning or release.

Validation

All green from a clean tree, and the tree stays clean after API regeneration + build + site build:

• pnpm typecheck · typecheck:guides (28 extracted / 20 no-check, up from 26 / 22) · typecheck:examples — pass
• pnpm lint:strict — pass
• pnpm test — 1846 pass (incl. the new link-drift test)
• pnpm build · verify:exports · verify:package · verify:create-exo-app (45 pass) — pass
• pnpm site:build (572 pages) · test:examples:smoke (118 pass) · astro check (0 errors) — pass
• API regeneration is deterministic; git status --short is clean after generate + build + site:build (only the intended 21 files)
• ⚠️ pnpm format:check flags 6 files (rollup.config.ts, scripts/extract-guide-snippets.ts, scripts/verify-create-exo-app.ts, test/site/example-search.test.ts, test/site/source-snippets.test.ts, tsconfig.guides.json) — all unmodified by this PR and pre-existing on main (prettier drift). Left untouched to avoid mixing unrelated reformatting into a docs PR; recommend a dedicated formatting/prettier-pin PR.

Release note

Release preparation must be recreated fresh from main after this PR. The stale release-prep PR #93 is superseded by #94 and this PR and can then be closed. This PR intentionally does not touch versioning or the changelog.