T0 + T1 + T2: engine redesign through new API surface (#1)

Implements tiers T0, T1, T2 of `docs/superpowers/specs/2026-04-23-trueskill-engine-redesign-design.md`. All three tiers have landed together on this branch because they build on one another; this PR rolls them up for a single review pass.

Per-tier plans:
- T0: `docs/superpowers/plans/2026-04-23-t0-numerical-parity.md`
- T1: `docs/superpowers/plans/2026-04-24-t1-factor-graph.md`
- T2: `docs/superpowers/plans/2026-04-24-t2-new-api-surface.md`

## Summary

### T0 — Numerical parity (internal)

- `Gaussian` switched to natural-parameter storage `(pi, tau)`; mul/div now ~7× faster (218 ps vs 1.57 ns).
- `HashMap<Index, _>` → dense `Vec<_>` keyed by `Index.0` (via `AgentStore<D>`, `SkillStore`).
- `ScratchArena` eliminates per-event allocations in `Game::likelihoods`.
- `InferenceError` seed type added (1 variant).
- 38 → 53 tests passing through T1.
- Benchmark: `Batch::iteration` 29.84 → 21.25 µs.

### T1 — Factor graph machinery (internal)

- `Factor` trait + `BuiltinFactor` enum (TeamSum / RankDiff / Trunc) driving within-game inference.
- `VarStore` flat storage for variable marginals.
- `Schedule` trait + `EpsilonOrMax` impl replacing the hand-rolled EP loop.
- `Game::likelihoods` rebuilt on the factor-graph machinery; iteration counts and goldens preserved to within 1e-6.
- 53 tests passing.
- Benchmark: `Batch::iteration` 23.01 µs (slight regression absorbed in T2).

### T2 — New API surface (breaking)

**Renames:**
- `IndexMap → KeyTable`, `Player → Rating`, `Agent → Competitor`, `Batch → TimeSlice`

**New types:**
- `Time` trait with `Untimed` ZST and `i64` impls; `Drift<T>`, `Rating<T, D>`, `Competitor<T, D>`, `TimeSlice<T>`, `History<T, D, O, K>` all generic.
- `Event<T, K>`, `Team<K>`, `Member<K>`, `Outcome` (`Ranked` variant; `#[non_exhaustive]`).
- `Observer<T>` trait + `NullObserver`.
- `ConvergenceOptions`, `ConvergenceReport`.
- `GameOptions`, `OwnedGame<T, D>`.

**Three-tier ingestion:**
- `history.record_winner(&K, &K, T)` / `record_draw(&K, &K, T)` — 1v1 convenience.
- `history.add_events(iter)` — typed bulk.
- `history.event(T).team([...]).weights([...]).ranking([...]).commit()` — fluent.

**Query API:** `current_skill`, `learning_curve`, `learning_curves` (keyed on `K`), `log_evidence`, `log_evidence_for`, `predict_quality`, `predict_outcome`.

**Game constructors:** `ranked`, `one_v_one`, `free_for_all`, `custom` — all returning `Result<_, InferenceError>`.

**`factors` module:** `Factor`, `Schedule`, `VarStore`, `VarId`, `BuiltinFactor`, `EpsilonOrMax`, `ScheduleReport`, `TeamSumFactor`, `RankDiffFactor`, `TruncFactor` now public.

**Errors:** `InferenceError` gains `MismatchedShape`, `InvalidProbability`, `ConvergenceFailed`; boundary panics converted to `Result`.

**Removed (breaking):** `History::convergence(iters, eps, verbose)`, `HistoryBuilder::gamma(f64)`, `HistoryBuilder::time(bool)`, `History.time: bool`, `learning_curves_by_index`, nested-Vec public `add_events`.

## Behavior change (documented in CHANGELOG)

`Time = Untimed` has `elapsed_to → 0`, so no drift accumulates between slices. The old `time=false` mode implicitly forced `elapsed=1` on reappearance via an `i64::MAX` sentinel — that quirk is not reproducible under a typed time axis. Tests that depended on it now use `History::<i64, _>` with explicit `1..=n` timestamps. One test (`test_env_ttt`) had 3 Gaussian goldens updated to reflect the corrected semantics; documented in commit `33a7d90`.

## Final numbers

| Metric | Before T0 | After T2 | Delta |
|---|---|---|---|
| `Batch::iteration` | 29.84 µs | 21.36 µs | **-28%** |
| `Gaussian::mul` | 1.57 ns | 219 ps | **-86%** |
| `Gaussian::div` | 1.57 ns | 219 ps | **-86%** |
| Tests passing | 38 | 90 | +52 |

All other Gaussian ops unchanged (~219 ps add/sub, ~264 ps pi/tau reads).

## Test plan

- [x] `cargo test --features approx` — 90/90 pass (68 lib + 10 api_shape + 6 game + 4 record_winner + 2 equivalence)
- [x] `cargo clippy --all-targets --features approx -- -D warnings` — clean
- [x] `cargo +nightly fmt --check` — clean
- [x] `cargo bench --bench batch` — 21.36 µs
- [x] `cargo bench --bench gaussian` — unchanged from T1
- [x] `cargo run --example atp --features approx` — rewritten in new API, runs clean
- [x] Historical Game-level goldens preserved in `tests/equivalence.rs`
- [x] Public API matches spec Section 4 (verified by integration tests in `tests/api_shape.rs`)

## Commit history

~45 commits total across T0 + T1 + T2. Each task is self-contained and individually tested; the branch is bisectable. See `git log main..t2-new-api-surface` for the full list.

## Deferred to later tiers

- `Outcome::Scored` + `MarginFactor` — T4
- `Damped` / `Residual` schedules — T4
- `Send + Sync` bounds + Rayon parallelism — T3
- N-team `predict_outcome` — T4
- `Game::custom` full ergonomics — T4

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: #1
Co-authored-by: Anders Olsson <anders.e.olsson@gmail.com>
Co-committed-by: Anders Olsson <anders.e.olsson@gmail.com>

CHANGELOG.md

@@ -2,6 +2,90 @@
 
 All notable changes to this project will be documented in this file.
 
+## Unreleased — T2 new API surface
+
+Breaking: every renamed type and the new public API land together per
+`docs/superpowers/specs/2026-04-23-trueskill-engine-redesign-design.md`
+Section 7 "T2".
+
+### Breaking renames
+
+- `Batch` → `TimeSlice`
+- `Player` → `Rating` (and the `.player` field on `Competitor` is now `.rating`)
+- `Agent` → `Competitor`
+- `IndexMap` → `KeyTable`
+- `History` field `.batches` → `.time_slices`
+
+### New types
+
+- `Time` trait with `Untimed` ZST and `i64` impls (generic time axis).
+- `Drift<T: Time>` — generified from the old `Drift` trait.
+- `Event<T, K>`, `Team<K>`, `Member<K>` — typed bulk-ingest event shape.
+- `Outcome` (`#[non_exhaustive]`) — `Ranked(SmallVec<[u32; 4]>)` with convenience
+  constructors `winner`, `draw`, `ranking`. `Scored` lands in T4.
+- `Observer<T: Time>` trait + `NullObserver` ZST — structured progress callbacks.
+- `ConvergenceOptions`, `ConvergenceReport` — configuration and post-hoc summary.
+- `GameOptions`, `OwnedGame<T, D>` — ergonomic Game constructors without lifetime
+  gymnastics.
+- `factors` module — re-exports `Factor`, `BuiltinFactor`, `VarId`, `VarStore`,
+  `Schedule`, `EpsilonOrMax`, `ScheduleReport`, and the three built-in factor types
+  (`TeamSumFactor`, `RankDiffFactor`, `TruncFactor`) as public API.
+
+### New `History` API
+
+- Three-tier ingestion:
+  - Tier 1 (bulk): `add_events<I: IntoIterator<Item = Event<T, K>>>(events) -> Result`
+  - Tier 2 (one-off): `record_winner(&K, &K, T)`, `record_draw(&K, &K, T)`
+  - Tier 3 (fluent): `event(T).team([...]).weights([...]).ranking([...]).commit()`
+- `converge() -> Result<ConvergenceReport, InferenceError>` — replaces
+  `convergence(iters, eps, verbose)`.
+- `current_skill(&K)`, `learning_curve(&K)`, `learning_curves()` (now keyed on `K`).
+- `log_evidence()` zero-arg, `log_evidence_for(&[&K])`.
+- `predict_quality(&[&[&K]])`, `predict_outcome(&[&[&K]])` (2-team only in T2;
+  N-team deferred to T4).
+- `intern(&Q)` / `lookup(&Q)` expose the internal `KeyTable<K>` for power users.
+- `History<T, D, O, K>` is now fully generic with defaults
+  `<i64, ConstantDrift, NullObserver, &'static str>`.
+
+### New `Game` API
+
+- `Game::ranked(&[&[Rating]], Outcome, &GameOptions) -> Result<OwnedGame, _>`.
+- `Game::one_v_one(&Rating, &Rating, Outcome) -> Result<(Gaussian, Gaussian), _>`.
+- `Game::free_for_all(&[&Rating], Outcome, &GameOptions) -> Result<OwnedGame, _>`.
+- `Game::custom(...)` minimal escape hatch for user-defined factor graphs
+  (`#[doc(hidden)]` — full ergonomics in T4).
+- `Game::log_evidence()` and `OwnedGame::log_evidence()` accessors.
+
+### Errors
+
+- `InferenceError` now carries `MismatchedShape { kind, expected, got }`,
+  `InvalidProbability { value }`, `ConvergenceFailed { last_step, iterations }`,
+  and `NegativePrecision { pi }`. Shape and bounds validation at the API boundary
+  now returns `Err` rather than panicking.
+
+### Removed (breaking)
+
+- `History::convergence(iters, eps, verbose)` — use `converge()`.
+- `HistoryBuilder::gamma(f64)` — use `.drift(ConstantDrift(g))`.
+- `HistoryBuilder::time(bool)` and `History.time: bool` — use the `Time` type parameter.
+- The nested-`Vec<Vec<Vec<_>>>` public `add_events` signature —
+  use typed `add_events(iter)`.
+- `learning_curves_by_index()` — use `learning_curves()`.
+
+### Performance
+
+`Batch::iteration` bench: **21.36 µs** (T1 was 22.88 µs on the same hardware, a
+~7% improvement from the typed-path being slightly more direct). Gaussian
+operations unchanged.
+
+### Notes
+
+- `Time = Untimed` returns `elapsed_to → 0` — **behavior change** from the old
+  `time=false` mode, which implicitly generated `elapsed=1` per event via an
+  `i64::MAX` sentinel in `Agent.last_time`. Tests that relied on the old
+  `time=false` semantics now use `History::<i64, _>` with explicit
+  `1..=n` timestamps.
+
 ## 0.1.0 - 2026-04-23
 
 ### Features