perf(web): load Shiki language grammars lazily instead of in the eager core chunk

[fanzeyi]

Related issue

N/A

Summary

• The manualChunks guard that keeps Shiki in one chunk (to avoid a cyclic
  import crash — the language-index ↔ alias-map split that throws "Cannot read
  properties of undefined (reading 'flatMap')" and blanks the Monaco/file
  viewer) matched both /shiki and /@shikijs/. That also swept every
  @shikijs/langs/<lang> grammar — which Shiki loads via dynamic import as
  per-language chunks — into the single, eagerly modulepreloaded core chunk.
  So ~200 language grammars (~1.68 MB gzip) were downloaded on every initial
  page load, even though a session uses only a few languages.
• Exclude @shikijs/langs/<lang> from the shiki chunk so grammars stay lazy
  per-language chunks. Keep Shiki's core, engines, and bundle glue together so
  the cyclic core stays intra-chunk — the engines must stay too: excluding them
  re-splits the cycle across chunks and reintroduces the flatMap crash.
• Initial-load eager JS drops from ~11.8 MB to ~4.37 MB (Shiki 1.68 MB → 466 KB
  gzip); grammars become 427 on-demand chunks. Layout-independent (same result
  under pnpm hoisted and isolated).

Test Plan

• pnpm --filter web run build succeeds.
• Verified the emitted shiki chunk statically imports only the rolldown
  runtime (no cross-chunk cycle) and contains bundledLanguagesAlias
  co-located with its reader — under both hoisted and isolated node_modules.
• Verified per-language grammar chunks (python, rust, typescript, …) are
  emitted separately and are NOT modulepreloaded by index.html.
• Recommended pre-merge smoke test: open the file viewer / Monaco editor and a
  markdown code block and confirm syntax highlighting renders.

Demo

N/A

Type of change

• Bug fix
• Feature
• UI / frontend change
• Refactor / chore
• Docs
• Test / CI
• Breaking change

Test coverage

• Unit tests added / updated
• Integration tests added / updated
• E2E tests added / updated
• Manual verification completed
• Existing tests cover this change
• Not applicable

Coverage notes

Verified via build output analysis: the shiki core chunk is acyclic with the
alias map co-located (cycle fix preserved), and language grammars are emitted as
separate, non-preloaded chunks. Existing Shiki/code-block tests exercise the
runtime highlighting path; this change only affects chunk grouping, not module
behavior.

Changelog

Faster initial load — syntax-highlighting language grammars now load on demand instead of all upfront.

Signed-off-by: Zeyi (Rice) Fan zeyi.f@databricks.com