bench,docs: capture T2 final numbers and update CHANGELOG

Batch::iteration: 21.36 µs (T1 was 22.88 µs on same hardware; ~7%
improvement attributed to the typed add_events(iter) path being
slightly more direct than the nested-Vec path it replaced).

Gaussian operations unchanged vs T1.

Full test suite: 90 green (68 lib + 10 api_shape + 6 game +
4 record_winner + 2 equivalence). No golden value changed across
the entire T2 tier.

CHANGELOG documents every breaking rename, every new public type,
and the two behavior changes (Untimed drift semantics, Result-based
boundary errors).

Closes T2 of docs/superpowers/specs/2026-04-23-trueskill-engine-redesign-design.md.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.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