The hub-client's project-rendering path was dropping user_grammars
because RenderToHtmlRenderer constructed its own per-page
RenderContext with no API for attaching a provider. Result: any qmd
under a `_quarto.yml` ancestor rendered fenced blocks for
user-grammar languages (e.g. the highlighting/03-user-grammar-toml
fixture) unhighlighted, even though the single-file render path was
wired correctly.

Migrate `RenderContext::user_grammar_provider` and the matching
`StageContext` field from `Option<Box<dyn UserGrammarProvider>>` to
`Option<Rc<RefCell<dyn UserGrammarProvider>>>` so a single handle
can be shared across every page a renderer touches without being
consumed. Add `RenderToHtmlRenderer::with_user_grammars(...)` and
install the provider on each per-page `RenderContext`. Update the
WASM project entry point `render_project_active_page_to_response`
to wrap the incoming `JsUserGrammars` in `Rc<RefCell<_>>` and pass
it through instead of dropping it.

The pipeline-internal `.take()` becomes `.clone()` so the `Rc` is
shared with the `StageContext` rather than transferred — matters
for multi-page renders that reuse the same renderer.

Plan + investigation notes:
`claude-notes/plans/2026-05-10-thread-user-grammars-renderer.md`.

Tests: new `crates/quarto-core/tests/render_to_html_user_grammars.rs`
drives `RenderToHtmlRenderer` through the orchestrator with a stub
provider and asserts the rendered HTML carries the matching
`<span class="hl-...">` wrapper. Confirmed it fails at HEAD without
the wiring and passes after.

End-to-end harness fixes folded in (without these the
highlighting/03-user-grammar-toml.qmd e2e fixture can't actually
exercise the fix in a browser):

- smokeAllDiscovery.ts: detect binary fixture extensions (.wasm
  grammars + common image/font types), read as bytes, base64-
  encode, and tag `contentType: 'binary'` with a matching
  mimeType so the Automerge sync client routes them as binary
  documents instead of corrupting them through utf-8 decoding.
  Add `contentType`/`mimeType` fields on `DiscoveredTest` and
  forward them in smoke-all.spec.ts.
- previewExtraction.ts::getPreviewHtml: derive a userGrammars
  context from VFS state (vfsListFiles + vfsReadFile /
  vfsReadBinaryFile, with the /project/ prefix stripped to match
  discoverUserGrammars's project-relative shape) so the re-render
  matches what Preview.tsx → automergeSync produces in the live
  iframe.

End-to-end verification (snippet from the iframe at assertion time):

  <pre class="sourceCode toml" …><code><span class="hl-property">
    <span class="hl-type">name</span> <span class="hl-operator">=</span>
    <span class="hl-string">"example"</span></span> …