refactor: one word per concept

Three vocabulary collisions, from #75.

**"rating" meant three things**, one the opposite of the exported type.
`Rating` is documented as static *configuration* — "this returns what it
was told", against every other accessor's "what inference inferred". But
`quality`'s parameter was `rating_groups: &[&[Gaussian]]` and its prose
said "rating groups" four times, where "rating" means a *posterior* — the
one thing `Rating` is documented not to be. Two error messages used it
that way too.

So a reader who learned `Rating = config` passed `Rating` values to
`quality`, which takes `Gaussian`; and one who learned "rating = what
comes out" was baffled that `h.rating(&k)` is not their skill.

"rating" is now reserved for the type. `quality(teams: &[&[Gaussian]])`,
and "every rating is finite" became "every posterior is finite".

**"agent" was a private fourth name for a competitor** — ~200 identifiers
against 236 uses of "competitor", and it leaked into two `pub` signatures
on `TimeSlice`. Now that #73 has made those internal this is a pure
rename, so the crate has one word for the entity throughout.

**"player" survived in one public signature** — `free_for_all(players:)`
plus two doc lines. Renamed, along with three internal closure bindings.
Doc examples that use "player" as a *key* are left alone: that is a
user's data, not the crate's vocabulary.

The panic-message expectations in tests/quality.rs moved with the prose,
which is the point of asserting on message text — the tests caught the
rename rather than papering over it.

Not touched: "performance" (always skill widened by beta), "skill",
"member" and "team" are each used for exactly one thing already.

Refs #75

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

tests/quality.rs

@@ -1,4 +1,4 @@
-//! `quality()` beyond two rating groups.
+//! `quality()` beyond two teams.
 //!
 //! The historical golden (two equal singletons) is asserted in
 //! `src/lib.rs::tests::test_quality`. These cover the N-group generalisation,
@@ -82,14 +82,14 @@ fn uneven_group_sizes_work() {
 }
 
 #[test]
-#[should_panic(expected = "at least 2 rating groups")]
+#[should_panic(expected = "at least 2 teams")]
 fn single_group_panics_with_clear_message() {
     let r = rating(25.0, 3.0);
     let _ = quality(&[&[r]], BETA);
 }
 
 #[test]
-#[should_panic(expected = "at least 2 rating groups")]
+#[should_panic(expected = "at least 2 teams")]
 fn zero_groups_panics_with_clear_message() {
     let _ = quality(&[], BETA);
 }