feat(snippet): embeddable <silk-snippet> element with live diagnostics, hover, and inlay hints

[julia-script]

Summary

Extracts the labs workbench editor into a new framework-free package, @silk-effect/snippet, exposing a <silk-snippet> custom element that any plain-HTML page can use with one script tag — and teaches the static documentation site to emit it for silk fences.

• packages/snippet — CodeMirror-based editor core (Editor.ts) with compiler-driven highlighting and opt-in language-server semantics: diagnostics, hover, inlay-hints, editable as boolean attributes. Snippets compile lazily (first visibility, main thread) as standalone modules with doctest's conventions, so readers see exactly what doctest verified. First consumer of LspDocument.inlayHints. Shadow-DOM theming via --silk-snippet-* custom properties with light/dark defaults; hover markdown rendered by a DOM renderer (HoverContent.ts) preserving link-scheme sanitization.
• Labs re-pointed — apps/docs/app/labs/editor.tsx is now a ~90-line React wrapper over the shared core; Hover.tsx deleted. Workbench palette mapped onto the element's CSS properties; observable behavior unchanged.
• Site emission — silk fences render as <silk-snippet diagnostics hover>; silk,ignore degrades to a highlight-only element that never compiles; other languages keep <pre><code>. The CLI ships the self-registering bundle (2.98 MB / ~714 KB gzip) with generated output, referenced relatively; silk-snippet:not(:defined) CSS keeps no-JS pages readable.
• Renderer boundary preserved — Site.render stays pure (bundle contents as an option); only the CLI shell resolves @silk-effect/snippet/bundle as an opaque asset. The site package's dependency guard tests were deliberately amended to allowlist exactly that, per their own comments.

OpenSpec: change extract-embeddable-silk-snippet-element implemented, synced (snippet-element, documentation-site-silk-snippets capabilities), and archived.

Test plan

• packages/snippet: 11 jsdom tests covering the spec scenarios (highlight-only never compiles, per-flag gating, doctest-parity diagnostics, unsafe hover links, read-only default, deferred compile, inlay hint widgets)
• packages/documentation-site: 27 tests including snippet emission, ignore degradation, bundle shipping, relative references
• Full suites green across snippet, documentation-site, docs, lsp, language; typecheck green
• Manual pass in Chromium against a statically served generated site: highlight, pass/fail diagnostics, hover tooltip, : SystemAllocator inlay hint, editable recompile-on-edit, light + dark themes