docs: complete the public API documentation contract

Closes the last open item in #25. `cargo clippy -W missing_errors_doc
-W missing_panics_doc -W must_use_candidate -W doc_markdown` went from 56
warnings to zero.

The 13 hand-written sections name the actual variants each function returns
rather than gesturing at "an error". Establishing that meant reading the error
paths — `Game::ranked` alone returns four distinct variants, and `record_draw`
can hit TieWithoutDrawProbability where `record_winner` provably cannot, since
a two-team decisive outcome has nothing to tie. Documenting those as
interchangeable would have been worse than leaving them undocumented, because
a reader would trust it.

Two existing doc comments already described panics in prose but not under a
`# Panics` heading, so neither rustdoc nor clippy surfaced them:
`Outcome::winner` and `EventBuilder::weights`. Both now carry the heading, and
`Outcome::winner` gained the note that it ties every loser, so `n >= 3` needs a
positive p_draw — the crate's easiest error to hit by accident.

The 43 mechanical fixes (31 `#[must_use]` on pure accessors, 11 missing
backticks) were applied with `cargo clippy --fix`. `#[must_use]` on Gaussian's
arithmetic and on `posteriors()` matters: discarding those results is always a
bug, and until now nothing said so.

Also documented why `[profile.release] debug = true` exists — cargo-flamegraph
needs the symbols, and library profile settings are ignored downstream, so it
reads as an oversight without the note.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T5SYDExxL4vZgvunrcNSMc

src/history.rs

@@ -198,6 +198,7 @@ impl Default for History<i64, ConstantDrift, NullObserver, &'static str> {
 }
 
 impl History<i64, ConstantDrift, NullObserver, &'static str> {
+    #[must_use]
     pub fn builder() -> HistoryBuilder<i64, ConstantDrift, NullObserver, &'static str> {
         HistoryBuilder::default()
     }
@@ -205,6 +206,7 @@ impl History<i64, ConstantDrift, NullObserver, &'static str> {
 
 impl<K: Eq + Hash + Clone> History<i64, ConstantDrift, NullObserver, K> {
     /// Like `builder()` but uses a custom key type `K` instead of the default `&'static str`.
+    #[must_use]
     pub fn builder_with_key() -> HistoryBuilder<i64, ConstantDrift, NullObserver, K> {
         HistoryBuilder {
             mu: MU,
@@ -552,7 +554,11 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
 
     /// 2-team win probability: returns `[P(team0 wins), P(team1 wins)]`.
     ///
-    /// Panics if `teams.len() != 2`. N-team support lands in T4.
+    /// N-team support lands in T4.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `teams.len() != 2`.
     pub fn predict_outcome(&self, teams: &[&[&K]]) -> Vec<f64> {
         assert_eq!(teams.len(), 2, "predict_outcome T2: 2 teams only");
         let gather = |team: &[&K]| -> Gaussian {
@@ -574,6 +580,15 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
     }
 
     /// Run the full forward+backward convergence loop and return a summary.
+    ///
+    /// Failing to reach `epsilon` within `max_iter` is not an error: the
+    /// returned report carries `converged: false` and the final step.
+    ///
+    /// # Errors
+    ///
+    /// `NonFiniteResult` if a sweep produces a NaN or infinite step. EP has
+    /// broken down at that point and further iterations cannot recover, so the
+    /// loop stops rather than reporting a NaN step as convergence.
     pub fn converge(&mut self) -> Result<ConvergenceReport, InferenceError> {
         use std::time::Instant;
 
@@ -830,6 +845,13 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
         Ok(())
     }
 
+    /// Record a single two-competitor event that `winner` won.
+    ///
+    /// # Errors
+    ///
+    /// Ingests through the same path as [`History::add_events`], so it returns
+    /// the same errors. A two-team decisive outcome cannot tie, so
+    /// `TieWithoutDrawProbability` is not reachable here.
     pub fn record_winner<Q>(&mut self, winner: &Q, loser: &Q, time: T) -> Result<(), InferenceError>
     where
         K: Borrow<Q>,
@@ -847,6 +869,13 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
         )
     }
 
+    /// Record a single two-competitor event that ended level.
+    ///
+    /// # Errors
+    ///
+    /// Ingests through the same path as [`History::add_events`]. Note
+    /// `TieWithoutDrawProbability` *is* reachable here: a draw needs a
+    /// positive `p_draw`.
     pub fn record_draw<Q>(&mut self, a: &Q, b: &Q, time: T) -> Result<(), InferenceError>
     where
         K: Borrow<Q>,
@@ -870,6 +899,17 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
     }
 
     /// Bulk-ingest typed events.
+    ///
+    /// # Errors
+    ///
+    /// - `MismatchedShape` if an event's outcome does not describe the same
+    ///   number of teams the event has, or if per-member weights do not match
+    ///   the team's membership.
+    /// - `InvalidParameter` if a per-event `score_sigma` override is not
+    ///   strictly positive.
+    /// - `TieWithoutDrawProbability` if an event ties two teams while the
+    ///   history's `p_draw` is zero. This includes `Outcome::winner(w, n)` for
+    ///   `n >= 3`, which ties every loser.
     pub fn add_events<I>(&mut self, events: I) -> Result<(), InferenceError>
     where
         I: IntoIterator<Item = crate::event::Event<T, K>>,