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.

bench/result.ts

@@ -0,0 +1,84 @@
+// The immutable result-record shape that the whole benchmark harness reads and
+// writes. One record captures one completed `(arm, task, trial)` run. Records
+// are never mutated after they are written; deepening a cell's sample size
+// appends new records rather than overwriting prior ones (see store.ts).
+//
+// The benchmark's own vocabulary (arm, cell, tier, cost-equivalent tokens) is
+// documented in bench/README.md and the benchmark-harness spec, deliberately
+// kept out of the tool's own domain glossary.
+
+/** The four tool conditions the benchmark compares. */
+export type Arm = "gitea-axi" | "tea" | "gitea-mcp" | "raw-api";
+
+/**
+ * The task tiers the scored suite is weighted across. Views group by tier to
+ * show where an arm wins or loses.
+ */
+export type Tier = "read" | "single-mutation" | "find-then-act" | "multi-step";
+
+/**
+ * The four token components retained per run. They are kept separate (rather
+ * than pre-summed) so cost-equivalent tokens can be re-weighted at render time
+ * without re-running — see the cost-equivalent-token-metric ADR. The auxiliary
+ * small model the runtime invokes for internal chores is folded into these
+ * counts, because it is real consumption against the same allowance.
+ */
+export interface TokenComponents {
+  /** Fresh, uncached input tokens (weighted 1x). */
+  freshInput: number;
+  /** Cache-creation (write) tokens. */
+  cacheCreation: number;
+  /** Cache-read tokens. */
+  cacheRead: number;
+  /** Output tokens. */
+  output: number;
+}
+
+/**
+ * Why a run failed. `incorrect` means the agent finished but the checker scored
+ * the outcome wrong; `confused` means it hit the turn cap; `hung` means it hit
+ * the wall-clock backstop. The confused-versus-hung split lets the reporting
+ * distinguish a lost agent from a stuck one.
+ */
+export type FailureTag = "incorrect" | "confused" | "hung";
+
+/** The pass/fail outcome of a run, tagged with the failure mode when it fails. */
+export type Outcome = { pass: true } | { pass: false; failure: FailureTag };
+
+/**
+ * One completed `(arm, task, trial)` run. Carries the metrics the headline and
+ * supporting views are computed from, plus the tags those views group by.
+ */
+export interface ResultRecord {
+  /** The arm under test. */
+  arm: Arm;
+  /** The task's stable identifier. */
+  taskId: string;
+  /** The task's tier. */
+  tier: Tier;
+  /** The trial index within the cell (1-based). */
+  trial: number;
+  /** ISO 8601 timestamp of when the sample was recorded. */
+  timestamp: string;
+
+  /** The four token components. */
+  tokens: TokenComponents;
+  /** Number of agent turns the run took. */
+  turns: number;
+  /** Wall-clock duration in milliseconds. */
+  durationMs: number;
+  /** The runtime's imputed cost in US dollars, retained as a secondary metric. */
+  imputedCostUsd: number;
+
+  /** Pass/fail outcome with a failure tag. */
+  outcome: Outcome;
+}
+
+/**
+ * The address of a cell in the sample store. A cell is one `(arm, task)` pair;
+ * its trials accumulate as samples within it.
+ */
+export interface CellKey {
+  arm: Arm;
+  taskId: string;
+}