refactor!: retire Index, intern and lookup

`Index` was public, `History::intern` and `History::lookup` returned
one, and no public method anywhere accepted one. It was a handle with
nowhere to go — and `key_table.rs` advertised the hot-path story it was
meant to enable ("power users can promote `&K` to `Index` and skip the
lookup"), which was never reachable through the public API.

It also shadowed `std::ops::Index`, which `CompetitorStore` implements,
so `use trueskill_tt::*` alongside `use std::ops::*` collided.

All three are `pub(crate)` now. `intern` stays internal because
ingestion needs it; `lookup` is gone entirely, since `current_skill`,
`rating` and `learning_curve` already answer "does this history know
this key" and all three take a borrowed key.

The three tests that used them asserted things a caller cannot observe.
They now assert what the interning bought:

- `record_winner_creates_two_competitors` compares posteriors instead of
  comparing two opaque indices for inequality.
- `intern_is_idempotent` becomes `a_repeated_key_is_one_competitor` — a
  key appearing in two events gives one competitor with a two-point
  learning curve, which is the observable form of the same claim.
- `lookup_returns_none_for_missing` becomes `an_unknown_key_is_unknown`.

Closes #73.

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

README.md

@@ -330,7 +330,7 @@ stay available at any size:
 
 Unknown keys are an error by default, not a silent omission: a team the history
 has never seen cannot produce a confident-looking probability. The error names
-the key, and every key must already be known — pre-filter with `lookup` or
+the key, and every key must already be known — pre-filter with
 `current_skill` if your caller cannot guarantee that.
 
 If predicting for competitors you have never seen is the point rather than a