Story-2025: wire the Brief field-history timeline to frame/versions

The Brief's per-field history control (the gutter History popover, the full-history modal, ⌘Z/⌘⇧Z, and Restore this version) used to show a single, current-only row: the brief payload carries only the newest frame version, so fieldVersionStack was an honest one-entry stub. The backend now serves the full append-only history at GET /app/matters/{id}/frame/versions (MatterFrameVersionHistoryResponse, newest-first). This story wires the timeline to that endpoint so every field shows its real multi-version history.

The read path (forum pattern)

• Browser proxy — new src/app/api/internal/matters/[matterId]/frame/versions/route.ts (GET), mirroring the sibling …/brief proxy: parse the matter id, call the gateway, map GatewayError to an ErrorResponse. The browser only ever talks to /api/internal/*; the gateway key + Clerk bearer stay server-side.
• Gateway — mattersGateway.frameVersions(matterId) fetches as unknown and runs it through assertContract(matterFrameVersionHistoryResponseSchema, …) (HR-21 — runtime contract validation at the trust boundary).
• Routing — a toBrowserRoute rule maps /app/matters/{id}/frame/versions → /api/internal/matters/{id}/frame/versions, placed before the bare /frame and /app/matters/{id} rules so it is never shadowed.
• Prefetch — the matter page.tsx prefetches the versions query into its HydrationBoundary alongside the brief and frame-status, so the first paint already carries history (no client refetch flash).

Mapping a version page to a field's stack

useFieldVersions(matterId, field, fallback) reads the shared versions query (TanStack dedupes the four call sites on the page into one request) and maps it with the pure mapFieldVersions(items, field) helper:

• A version's frame content lives at envelope.frame[field] — the MatterFrameEnvelope (the brief flattens it; the version projection keeps it nested). The helper reads envelope.frame ?? envelope so it is robust to either shape.
• value = textField(frame, field) (same accessor the current value renders through), author = authorDisplayName ?? (agentRunId ? "Vega" : "Counsel"), actorKind from agentRunId, id = `${field}:v${version}`.
• Drop empties (a version where this field is blank is not a row) and collapse consecutive identical values (a row only appears when this field actually changed — a summary edit that didn't touch core_dispute adds no core-dispute row). Input is newest-first, so output is too.
• While the query loads — or when a field has no non-empty history yet — the hook returns the current-only fallback (fieldVersionStack), so the timeline never flashes blank on a field that already shows a value.

Wiring

matterId threads from matter-brief-page.tsx (matter.matterId) into BriefHeader (frame_summary) and the EditableFields (core_dispute, theory_of_case, settlement_posture). Each builds its versions from useFieldVersions instead of the sync stub. The EditableFieldWithHistory presentation component is unchanged — it already renders any newest-first SummaryVersion[], so undo/redo and Restore work across the real versions for free.

Verification

pnpm lint (0 errors), pnpm build (green; the new /api/internal/matters/[matterId]/frame/versions route is registered), and a new presentation.test.ts covering mapFieldVersions (order, dedupe/drop-empty, agent-vs-counsel authors, flattened-envelope fallback) all pass.