refactor!: remove the factor-graph surface nothing used, add try_winner

Two decisions taken before cutting 0.5.0.

#42 — the `Schedule` trait was public API the engine never called. Its
only call site was `Game::custom`, itself `#[doc(hidden)]`, and
`EpsilonOrMax` was never constructed anywhere. Removing that surface
showed the problem was larger than the issue described: with `custom`
gone, the compiler found `Factor`, `BuiltinFactor`, `RankDiffFactor` and
`TeamSumFactor` all dead too.

`Game::run_chain` drives a local `DiffFactor` enum and bypasses the
whole T1 abstraction — it has done since it was written. So this is not
just an unused extension point but the machinery it was built on, and
`CLAUDE.md` was documenting it as live architecture.

Removed: `graph` module, `Schedule`, `EpsilonOrMax`, `ScheduleReport`,
`Game::custom`, `Factor`, `BuiltinFactor`, `RankDiffFactor`,
`TeamSumFactor`. `TruncFactor`, `MarginFactor`, `VarStore` and `VarId`
stay — inference uses those. The measurement behind choosing removal
over wiring is in #42: the within-game loop converges in 1 to 8
iterations against a cap of 30, so a `Residual` schedule has no headroom
to reclaim, and `Damped` already shipped as `ConvergenceOptions::alpha`.

#20 — `Outcome::winner` panicking on an out-of-range index. Kept, and
the reasoning is now on the method. It is the only constructor here that
validates, which looks inconsistent until you try deferring like its
siblings: `winner(5, 2)` produces ranks `[1, 1]`, an all-tied draw that
ingestion accepts without complaint when `p_draw > 0`. Asking "team 5
won" and silently getting "everyone drew" is the exact failure this
crate keeps removing, so the check belongs where the mistake is.

Adds `Outcome::try_winner` for indices that are computed or parsed
rather than written literally, following the `new`/`try_new` convention.
That is additive; the panicking form stays because every call site in
this repo, its tests and its README passes literals, where a `?` would
be noise.

BREAKING CHANGE: the `graph` module and everything it exported are
removed, as are `Game::custom` and `ScheduleReport`.

Closes #42. Closes #20.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011hcFjNDmHXZF8URGLku5zZ

CLAUDE.md

@@ -80,11 +80,11 @@ History  →  TimeSlice[]  →  Event[]  →  Item[]
   `tau = mu/sigma²`). `Mul`/`Div` are the EP product/cavity: pure adds and
   subtracts. Variance-space ops (`Add`, `Sub`, `exclude`, `forget`) go through
   `from_mv`/`variance()` and take no square root.
-- **`factor/`** — `TeamSumFactor`, `RankDiffFactor`, `TruncFactor` (ranked),
-  `MarginFactor` (scored), over a flat `VarStore`. `BuiltinFactor` dispatches
-  by enum rather than `dyn`.
-- **`Schedule`** (`schedule.rs`) — drives factor propagation. `EpsilonOrMax` is
-  the only implementation.
+- **`factor/`** — `TruncFactor` (ranked) and `MarginFactor` (scored) over a
+  flat `VarStore`. `Game::run_chain` drives them directly through a local
+  `DiffFactor` enum; there is no `Schedule` indirection and no generic `Factor`
+  trait. Both were removed once measurement showed nothing had ever used them
+  — see #42.
 - **`Competitor`** (`competitor.rs`) — per-history temporal state (`message`,
   `last_time`). **`Rating`** (`rating.rs`) — static config (prior, `beta`, drift).
 - **`storage/`** — `SkillStore` (per slice, `pub(crate)`) and `CompetitorStore`