Mobile lag: Shiki grammar registry bundles 2.3MB of JS into workspace-checkout-display, no service worker for caching #1063

Implementation Plan

We'll tackle items 3-5 from the issue body:

3. Tree-shake the grammar registration map — bundle only the top 15-20 languages
4. Split workspace-checkout-display into editor-only vs viewer-only chunks
5. Add bundle size budget CI check

Phase 1: Audit

• Identify where the Shiki grammar registry is constructed in the bb source
• Determine which languages are actually used on critical paths (thread view vs editor)
• Measure current chunk sizes as baseline

Phase 2: Tree-shake grammars (#3)

• Extract the grammar registration map into a separate file with explicit imports for the top 15-20 languages
• Make remaining 150+ languages truly on-demand (dynamic import when first requested, not eagerly registered)
• Verify workspace-checkout-display shrinks substantially

Phase 3: Split workspace chunks (#4)

• Separate the viewer-only code path from the full editor path
• Lazy-load the Shiki engine only when the user opens a file for editing
• Verify the thread-view route doesn't pull in Shiki at all

Phase 4: Bundle budget CI (#5)

• Add bundle size budget to Vite config (or CI step)
• Set thresholds: 500KB max per chunk uncompressed, 1.5MB total initial payload
• Wire into CI so PRs that regress bundle size fail

Phase 5: Verify

• Re-measure workspace-checkout-display.js size
• Re-test on iPhone via Safari Web Inspector Timelines
• Confirm click latency under 50ms parse/compile time