TypeAgent Studio (part 5): two-mode Impact Report replay

[TalZaccai]

Summary

Stacked on #2553 (part 4). Makes the Impact Report replay's deterministic dispatch model explicit instead of implicitly mixing two paths that never coexist in a real dispatcher config.

Two modes, surfaced as a toggle in the report's action bar:

• nfa-grammar (default) — both versions match against the compiled grammar only; the construction cache is never consulted, so A and B stay symmetric. The cleanest signal for what a grammar/schema edit changed. Faithful to the dispatcher's nfa grammar system.
• completionBased-cache (opt-in) — the working-tree side consults the live construction cache before the grammar (the default-dispatcher path). A B-side cache hit reflects what the dispatcher would serve, but the run is asymmetric.

Changes

Level A — core gating

• Add StudioReplayMode = "nfa-grammar" | "completionBased-cache" and mode? on StudioReplayRequest (default nfa-grammar).
• Gate the live construction-cache consult behind completionBased-cache. Default runs are now grammar-only and A/B-symmetric; the construction-cache method label no longer auto-appears.

Level B — plumbing + UI + test

• Thread mode through the webview run message; parseWebviewMessage validates it (unknown/missing → nfa-grammar). The host forwards it into replayCorpus; the channel/RPC layers ride on StudioReplayRequest unchanged.
• Add a two-state Grammar ⇄ Cache toggle to the action bar with explanatory tooltips, persisted/restored alongside the version selection and disabled with the other controls during a run.
• Add an injectable resolveConstructionCache seam to CreateStudioRuntimeOptions (mirrors replayResolver/collisionScanner) so the gating is testable without a live cache.

Testing

• pnpm run build typeagent-core — green
• Studio suite — 184/184
• Core replay specs (replay|grammarReplay|constructionCache) — 40/40
• 3 new runtime gating tests: cache skipped in nfa-grammar (and when mode omitted), consulted in completionBased-cache
• esbuild webview bundle clean; prettier clean

Notes

• grammarResolver.ts / constructionCacheResolver.ts unchanged — the gate lives at the runtime call site.
• Next fidelity rung (separate slice): L4a — stand up one live working-tree agent and run real validateWildcardMatch/validateEntityWildcardMatch.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com