docs: document the whole public surface and deny(missing_docs)

80 undocumented public items, including three that are first contact:
`History::current_skill` — the method the crate's own first example calls
— `EventBuilder`, the type `h.event(t)` hands you, and `Gaussian::mu()`.
Now zero, and `#![deny(missing_docs)]` keeps it that way.

Several docs are measurements rather than readings of the code:

- `Outcome::Ranked` says ranks are used ordinally, so `[0, 1, 2]` and
  `[0, 5, 90]` are the same observation. Measured: bit-identical
  posteriors for both.
- `OwnedGame::log_evidence` says two identically-rated competitors give
  exactly `ln(0.5)`. Written as a doctest, so it runs.
- `Member::weight` says zero and negative are accepted. Measured.
- `ConvergenceReport::final_step` is `(|Δmu|, |Δsigma|)` in skill units,
  NOT natural parameters. That one had to be traced through
  `Gaussian::delta` rather than assumed from the neighbouring vocabulary.
- `GameOptions::score_sigma` rejects non-positive and NaN but accepts
  `+inf`, which is what the guard actually says.

README: it is the front door for a crate on a private registry, and it
opened with a link dump followed by 130 lines on drift. The first
`record_winner → converge → current_skill` block was at line 226 of 307.
It now leads with what the crate is, an install line, a quickstart, a
"which entry point?" table, and the `converge`-is-strict rationale that
was the crate's most opinionated recent decision and went unmentioned.
The two canonical examples disagreed on spelling (`History::default()`
vs `History::builder().build()`, `current_skill("a")` vs
`current_skill(&"a")`); they now agree. Five new README blocks are
doctested, taking the suite from 19 to 25.

`pub use smallvec;`. Four public items name `SmallVec` in their
signatures, and the only `Joint` example failed to compile from a
consumer crate with `unresolved import smallvec` — the dependency was in
the API but not reachable. Both worked examples now use the re-export,
so they teach the path that works downstream.

Vocabulary, from #75: "agent" was a fourth word for competitor, 200
occurrences, and it had reached public signatures before #73 un-exported
`TimeSlice`. Now zero.

Closes #77. Refs #75.

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

src/event_builder.rs

@@ -9,6 +9,33 @@ use crate::{
     time::Time,
 };
 
+/// One match under construction, handed back by [`History::event`].
+///
+/// Describes a single event a piece at a time — teams, then per-member weights
+/// if they differ, then how it ended — instead of assembling an
+/// [`Event`] value and passing it to [`History::add_events`]. The two routes
+/// ingest through the same chokepoint and accept the same things; this one just
+/// reads better for a single match written by hand.
+///
+/// The builder borrows the history mutably and nothing reaches it until
+/// [`EventBuilder::commit`]. A builder that is dropped instead ingests
+/// nothing at all, silently — hence the `#[must_use]`, which is the only
+/// warning you get. `commit` is also where validation surfaces: the setters
+/// return `Self` to keep the chain fluent, so a mismatch such as a weight list
+/// the wrong length is recorded while building and returned as an error from
+/// `commit`.
+///
+/// ```
+/// # use trueskill_tt::History;
+/// let mut h = History::builder().build();
+/// h.event(1)
+///     .team(["alice", "bob"])
+///     .team(["carol"])
+///     .ranking([0, 1])
+///     .commit()?;
+/// assert_eq!(h.event_count(), 1);
+/// # Ok::<(), trueskill_tt::InferenceError>(())
+/// ```
 #[must_use = "an event is only recorded by `.commit()`; a dropped builder \
               silently ingests nothing"]
 pub struct EventBuilder<'h, T, D, O, K>