Docs snippet-check hardening (AST-based)

[truthixify]

Extra labels: ci, dx
Tier: M (2-4 days) | Type: ci

Context. The snippet-check gate compiles each fence in isolation. That catches syntax errors but misses import-resolution errors. Move to a small monorepo that installs the current SDK + tries to type-check each snippet against real types.

Scope.

• scripts/snippet-check/ sets up a project per language (ts + tsx).
• Fetches SDK from workspace or latest published.
• Runs tsc --noEmit per snippet.
• Preserves the no-check opt-out for pseudo-code.

Acceptance.

• Real broken import in a fence fails CI
• Existing 438 passing fences still pass
• Wall-clock still < 3 minutes

Files. scripts/snippet-check/, .github/workflows/snippets.yml.