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/time_axis.rs

@@ -94,7 +94,7 @@ fn a_custom_time_type_and_a_custom_drift_work_together() {
     }
     assert!(h.converge().unwrap().converged);
 
-    let curve = h.learning_curve(&"veteran");
+    let curve = h.learning_curve(&"veteran").unwrap();
     assert_eq!(curve.len(), 4, "one point per season: {curve:?}");
     for (season, g) in &curve {
         assert!(
@@ -148,5 +148,5 @@ fn new_constructs_on_any_axis_directly() {
     h.record_winner(&"a".to_string(), &"b".to_string(), Season(7))
         .unwrap();
     assert!(h.converge().unwrap().converged);
-    assert_eq!(h.learning_curve("a")[0].0, Season(7));
+    assert_eq!(h.learning_curve("a").unwrap()[0].0, Season(7));
 }