feat: add benchmark aggregator and reporting (task 0030)

Add bench/aggregate.ts: the pure aggregator seam that rolls the
accumulated sample store into a readable comparison. aggregate produces
a headline table (one row per arm, cost-equivalent tokens as the
headline plus raw tokens, turns, duration, success rate, coverage, and
imputed cost as a de-emphasized secondary column), per-tier and
per-token-component breakdowns, and the separate bonus table; renderReport
renders it as stable text. Cost-equivalent tokens are weighted at render
time from the four retained components per ADR 0014, and incomplete
coverage is annotated rather than hidden. Unit-tested against synthetic
append-only sample stores.

.claude/tasks/0030-bench-aggregator-and-reporting.md

@@ -13,8 +13,18 @@ Supporting views derived from the same records include a per-tier breakdown, a p
 
 ## Acceptance criteria
 
-- [ ] The headline table renders one row per arm with cost-equivalent tokens as the headline, plus raw tokens, turns, duration, success rate, coverage, and imputed cost as a de-emphasized secondary column.
-- [ ] Cost-equivalent tokens are computed from the retained four components at render time using the documented pricing-ratio weights.
-- [ ] A partial matrix renders without error and incomplete coverage is annotated rather than hidden or treated as complete.
-- [ ] Per-tier and per-token-component breakdowns and the separate bonus table are rendered from the same records.
-- [ ] Rendering is stable and unit-tested against a synthetic append-only sample store.
+- [x] The headline table renders one row per arm with cost-equivalent tokens as the headline, plus raw tokens, turns, duration, success rate, coverage, and imputed cost as a de-emphasized secondary column.
+- [x] Cost-equivalent tokens are computed from the retained four components at render time using the documented pricing-ratio weights.
+- [x] A partial matrix renders without error and incomplete coverage is annotated rather than hidden or treated as complete.
+- [x] Per-tier and per-token-component breakdowns and the separate bonus table are rendered from the same records.
+- [x] Rendering is stable and unit-tested against a synthetic append-only sample store.
+
+## Implementation Notes
+
+- The aggregator lives in `bench/aggregate.ts` as a pure seam: `aggregate` rolls a flat record list up against the task definitions into a `Report` (headline, per-tier and per-token-component breakdowns, and the bonus table), and `renderReport` renders that report as stable text. `readAllSamples(store)` drains a `SampleStore` into the record list the aggregator consumes, so the whole pipeline is `renderReport(aggregate({ records: readAllSamples(store), suite, bonus }))`.
+- Cost-equivalent tokens are computed at render time from `COST_EQUIVALENT_WEIGHTS` (fresh input 1×, cache-write 1.25×, cache-read 0.1×, output 5×, per ADR 0014). The 1.25× cache-write weight is the 5-minute-TTL multiplier: the stored record retains a single un-TTL'd `cacheCreation` component, so the default write price applies. Documented at the constant.
+- `aggregate` takes the suite as `TaskCoverage = Pick<BenchTask, "id" | "tier">` rather than the full `BenchTask`, since the aggregator only needs each task's id (to key cells) and tier (to group), never its scoring function. The real scored suite satisfies this, and tests can pass bare `{ id, tier }` lists. Coverage is scored against the full suite per arm, so a cell is `covered` at or above the reporting floor, `partial` below it, and `missing` at zero — the three always sum to the task count, which is what keeps a half-run matrix from reading as complete.
+- Metrics are per-run means over an arm's samples; every mean is `number | null`, with `null` (rendered as an em dash) for an arm or cell with no samples, so an unrun arm is never shown as a zero. Success rate is the passing fraction.
+- The bonus table renders one row per bonus definition carrying its capability metadata (operation, direction, note, and gitea-axi's own applicability), plus per-arm run metrics only for arms that actually have samples for that bonus cell — usually none, since the runner drives only the scored suite. This is the honest "from the same records" reading without inventing per-arm applicability the `BonusTask` model does not carry.
+- Scope: this task delivers the aggregator seam only. No CLI wrapper was built — `bench/` slices split their command wiring into their own tasks (task 0029 was the dedicated run-loop CLI), so a `bench:report` command over this seam is flagged as a natural follow-up in `bench/README.md` rather than folded in here. All five acceptance criteria are about the pure aggregator, which is fully implemented and unit-tested (9 tests, including an append-order-stability test driving two real `createSampleStore` instances).
+- Review (three-axis, `/review-uncommitted`): Risk overall Low. Spec axis clean — faithful and complete, weights correct per ADR 0014, coverage annotated not hidden. Standards axis found no hard violations, only judgement-call smells (Duplicated Code / Data Clumps / Repeated Switches across the four aggregation passes); left as deliberate trade-offs — the reviewer noted the flat per-pass form is readable and the view interfaces genuinely diverge, and the shared cost-equivalent-mean was already factored into `meanCostEquivalent`.