fix: reject malformed events at the ingestion boundary

A one-team event reached `run_chain`, which builds one diff link per
adjacent pair of teams, leaving it to index `links[1..]` on an empty
vector. That panicked with "range start index 1 out of range for slice
of length 0" — from `History::add_events`, in a release build, through
entirely safe API.

An empty team was the quieter half of the same gap. It contributes no
performance, so a malformed event converged and handed back a finite,
plausible-looking posterior for whoever it was matched against. That is
this crate's characteristic defect: a public surface reporting a
constant that looks like an answer.

A non-finite score was the third. `converge` did report NonFiniteResult,
so it was detected — but a caller reading `current_skill` before
converging was handed `tau: NaN` with nothing to say so.

`NotEnoughTeams` and `EmptyTeam` already existed. They were checked on
the prediction paths and nowhere else, which is exactly why ingestion
could still manufacture the states they describe. The checks go in
`add_events_with_prior` alongside the tie check, for the same reason
that one is there: every ingestion route lands on it, so `record_winner`,
`record_draw` and `EventBuilder` inherit them rather than each needing
their own.

Also corrects documentation that had been stating the opposite of the
code since 8c087ad in 0.4.0. README.md and the `with_prior` /
`with_drift_scale` doc comments all still said competitor configuration
was "captured at first appearance" and had "no effect" on a known key.
It now applies whenever supplied and refits the whole history. A reader
would have concluded late configuration was impossible and built a
workaround for a limitation that does not exist. CI compiles README code
blocks but not prose, which is why it survived three releases.

The comment in tests/degenerate_inputs.rs claiming a one-team event was
"rejected for an unrelated reason" was wrong when written — it panicked.

Refs #18, #26

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

README.md

@@ -134,10 +134,15 @@ h.add_events(vec![Event {
 h.converge().unwrap();
 ```
 
-Like `with_prior`, the scale is **competitor configuration captured at first
-appearance** — setting it on a key the history already knows has no effect. It
-must be finite and non-negative; ingestion otherwise fails with
-`InferenceError::InvalidParameter`.
+Like `with_prior`, the scale is **competitor configuration, not a per-event
+value**: it applies to the competitor for the whole history, and it applies
+whenever it is supplied — including on a key the history already knows.
+Configuring one late still refits the whole history rather than taking effect
+only from that event onward, because `converge` refits from competitor state.
+Repeating the same value is inert; supplying two *different* values for one
+competitor within a single batch is `InferenceError::ConflictingCompetitorConfig`,
+since events in a batch have no order. The scale must be finite and
+non-negative; ingestion otherwise fails with `InferenceError::InvalidParameter`.
 
 Note that the fluent `EventBuilder` (`h.event(t).team([...])`) sets weights but
 not `drift_scale` or `prior`; those need the typed `Event` / `Team` / `Member`