## Summary
Cancel, replay, reschedule, metadata, tags, and idempotency-key-reset
now succeed against a run that's still in the mollifier buffer.
Mutations are applied to the buffered snapshot via Lua CAS; the drainer
carries the mutation forward when it replays.
Primitives added:
- `mutateWithFallback` — PG-first / buffer-fallback resolver with
bounded-wait safety net for entries that transition mid-mutation.
- `applyMetadataMutation` — buffered metadata PUT mirroring the PG-side
retry loop with CAS atomicity.
- `resolveRunForMutation` — discriminated-union resolver used by route
`findResource` so the route builder's pre-action 404 check sees buffered
runs.
Routes wired (whole files, no GET/POST splits):
- `api.v2.runs.\$runParam.cancel.ts`
- `api.v1.runs.\$runParam.replay.ts`
- `api.v1.runs.\$runParam.reschedule.ts`
- `api.v1.runs.\$runId.metadata.ts`
- `api.v1.runs.\$runId.tags.ts`
- `resetIdempotencyKey.server.ts`
Stacked on the reads PR.
## Test plan
- [x] \`pnpm run typecheck --filter webapp\` passes
- [x] \`pnpm run test --filter webapp
test/mollifierMutateWithFallback.test.ts\` passes
- [x] \`pnpm run test --filter webapp
test/mollifierApplyMetadataMutation.test.ts\` passes
- [x] \`pnpm run test --filter webapp
test/mollifierResolveRunForMutation.test.ts\` passes
---------
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Chris Arderne