feat: add benchmark scaffold and result store (task 0022)

Lay the foundation the benchmark harness reads and writes: a bench/
directory (excluded from the published npm package), the immutable
result-record shape, and an append-only per-cell sample store.

- bench/result.ts: the ResultRecord shape — four token components,
  turns, duration, imputed cost, tagged pass/fail outcome, and the
  arm/task/tier/trial/timestamp tags. Arm and Tier are typed unions.
- bench/store.ts: append-only sample store, one JSONL file per cell at
  <root>/<arm>/<taskId>.jsonl; deepening a cell only ever adds samples.
- bench/README.md: harness working docs and benchmark vocabulary, kept
  out of the tool's domain glossary per the spec.
- Dedicated bench test tier (vitest.bench.config.ts, npm run test:bench)
  kept out of the fast tier; tsconfig typechecks bench.
- Packaging tier asserts bench/ never ships in the tarball.

.claude/tasks/0022-bench-scaffold-and-result-store.md

@@ -12,7 +12,16 @@ The store appends records as immutable, timestamped samples to a per-cell locati
 
 ## Acceptance criteria
 
-- [ ] A `bench/` directory exists and is excluded from the npm package (verified by the packaging tier or an equivalent `files` check).
-- [ ] The result-record shape records the four token components, turns, duration, imputed cost, outcome, failure tag, and the arm/task/tier/trial/timestamp tags.
-- [ ] Appending a sample to a cell that already has samples leaves the prior samples intact; reading the cell returns all of them.
-- [ ] A round-trip test writes several samples across cells and reads back exactly what was written.
+- [x] A `bench/` directory exists and is excluded from the npm package (verified by the packaging tier or an equivalent `files` check).
+- [x] The result-record shape records the four token components, turns, duration, imputed cost, outcome, failure tag, and the arm/task/tier/trial/timestamp tags.
+- [x] Appending a sample to a cell that already has samples leaves the prior samples intact; reading the cell returns all of them.
+- [x] A round-trip test writes several samples across cells and reads back exactly what was written.
+
+## Implementation Notes
+
+- The harness lives in `bench/`, kept out of the published package by the existing `files` allow-list (`["dist", "skills"]`); a new assertion in the packaging tier (`test/packaging/packaging.test.ts`) locks in that `package/bench` never ships.
+- `bench/result.ts` holds the immutable `ResultRecord` shape. `Arm` and `Tier` are typed unions (the four arms; the four task tiers from the spec) rather than bare strings, and the failure tag is `"incorrect" | "confused" | "hung"` — the spec/task only required distinguishing confused (turn cap) from hung (wall-clock backstop), and `incorrect` is added for the ordinary checker-scored-wrong failure the later runner will record.
+- `bench/store.ts` is the append-only sample store, backed by one newline-delimited JSON file per cell at `<root>/<arm>/<taskId>.jsonl`. Immutability is structural: the store exposes only `append`/`read`/`cells`, and append is a bare file append, so deepening a cell can only add lines. `cells()` enumerates written cells — slightly beyond the literal criteria but foundational for the aggregator slice (0030), which reads the store.
+- Bench tests run in a dedicated tier (`vitest.bench.config.ts`, `npm run test:bench`), colocated with the source, kept out of the fast tier so harness code never counts against the `src/` coverage thresholds. `tsconfig.json` now includes `bench` so the harness is typechecked.
+- Benchmark vocabulary (arm, cell, tier, cost-equivalent tokens, seed, checker) is documented in `bench/README.md`, deliberately kept out of the tool's domain glossary (`.claude/CONTEXT.md`) per the spec's Further Notes.
+- Review: Risk overall Low; Spec axis clean; two Standards judgement-call Duplicated Code findings addressed in the refactor — `append` now routes through `cellPath`, and the repeated ENOENT handling is factored into one `ignoreEnoent` helper.