fix!: per-key queries report unknown keys instead of a plausible constant

Two accessors answered a question about a key the history had never seen
with a well-formed value indistinguishable from a real answer.

`log_evidence_for` filter_map'd unknown keys away. An empty target list
means "no restriction" downstream, so a list of *entirely* unknown keys
returned the whole-history evidence: measured on a two-cohort fixture,
`log_evidence_for(["typo"])` returned exactly `log_evidence()`. On the
one workload it is documented for — leave-one-out cross-validation —
that is the un-held-out score, a plausible number that silently
invalidates the comparison it was computed for. It now returns
`Err(UnknownKey)` naming the offending position.

`learning_curve` and `filtered_learning_curve` returned an empty `Vec`
both for a typo'd key and for a competitor who is registered but has not
played yet. They now return `Option`, so `None` is "never heard of it"
and `Some(vec![])` is "known, no appearances".

Tests carry a control case in each direction, so they cannot pass by
everything returning the same thing.

Closes #66, closes #70.

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

tests/registration.rs

@@ -119,7 +119,7 @@ fn registration_reaches_a_competitor_first_seen_through_record_winner() {
     assert_eq!(rating.prior().mu(), PINNED.mu());
 
     // Pinned means pinned: no drift across the two slices.
-    let curve = h.learning_curve(&"layout");
+    let curve = h.learning_curve(&"layout").unwrap();
     assert!(curve.len() >= 2);
     let widest = curve
         .iter()