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>

benches/baseline.txt

@@ -65,3 +65,36 @@ Gaussian::pi_tau_combined 234.xx ps    (unchanged)
 # - Gaussian operations unchanged vs T0.
 # - All 53 tests pass. factor graph infrastructure (VarStore, Factor trait,
 #   BuiltinFactor, TruncFactor, EpsilonOrMax schedule) in place for T2.
+
+# After T2 (2026-04-24, same hardware)
+
+Batch::iteration           21.36 µs   (1.07× vs T1 22.88 µs — 7% improvement)
+Gaussian::add             218.97 ps    (unchanged)
+Gaussian::sub             218.58 ps    (unchanged)
+Gaussian::mul             218.59 ps    (unchanged)
+Gaussian::div             218.57 ps    (unchanged)
+Gaussian::pi              264.20 ps    (unchanged)
+Gaussian::tau             260.80 ps    (unchanged)
+
+# Notes:
+# - API-only tier; hot inference path unchanged. The 7% improvement on
+#   Batch::iteration likely comes from the typed add_events(iter) path
+#   being slightly more direct than the nested-Vec path it replaced
+#   (one less layer of composition construction per event).
+# - Public surface now matches spec Section 4:
+#     record_winner / record_draw / add_events(iter) / event(t).team().commit()
+#     converge() -> Result<ConvergenceReport, InferenceError>
+#     learning_curve(&K) / learning_curves() / current_skill(&K)
+#     log_evidence() / log_evidence_for(&[&K])
+#     predict_quality / predict_outcome
+#     Game::ranked / one_v_one / free_for_all / custom
+#     factors module (pub Factor/Schedule/VarStore/EpsilonOrMax/BuiltinFactor)
+# - Breaking type renames: Batch→TimeSlice, Player→Rating, Agent→Competitor,
+#   IndexMap→KeyTable.
+# - Generic over T: Time (default i64), D: Drift<T>, O: Observer<T>,
+#   K: Eq + Hash + Clone (default &'static str).
+# - Legacy removed: History::convergence(iters, eps, verbose),
+#   HistoryBuilder::gamma(), HistoryBuilder::time(bool), History::time field,
+#   learning_curves_by_index(), nested-Vec public add_events().
+# - 90 tests green: 68 lib + 10 api_shape + 6 game + 4 record_winner +
+#   2 equivalence.