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/time_slice.rs

@@ -370,6 +370,13 @@ impl<T: Time> TimeSlice<T> {
             .collect::<HashMap<_, _>>()
     }
 
+    /// Sweep this slice's events once, starting at index `from`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if an event references a competitor with no entry in this
+    /// slice's skill store. `add_events` inserts one for every participant, so
+    /// this cannot happen for slices built through the public API.
     pub fn iteration<D: Drift<T>>(&mut self, from: usize, agents: &CompetitorStore<T, D>) {
         if from == 0 && self.color_groups_dirty {
             self.recompute_color_groups();