fix: close out four small issues and pin #27's repro

#29 — log_evidence and log_evidence_for took &mut self while mutating
nothing. Loosening them to &self is not source-breaking for ordinary callers
(a &mut reborrows as & transparently) and brings them in line with the
filtered_* accessors added last week.

Not the mechanical change it looked like: under the rayon feature the closure
in log_evidence_internal captured all of &self rather than just the competitor
store, which drags KeyTable<K> in and demands K: Sync from every caller. That
compiled while the method took &mut self and stopped compiling the moment it
did not. Binding `let agents = &self.agents;` before the closure narrows the
capture; the comment there says why, because the next person to inline it will
reintroduce the bound.

#31 — TimeSlice::add_events constructed Skill with ..Default::default() while
filtered_step spells every field out. The design relies on a new Skill field
being a compile error at construction sites rather than a silent default, and
that tripwire only fired at one of the two. Now both.

#28 — log_evidence_internal's `forward` flag is a genuine forward-only
quantity only on a history that has never been converged, because iteration
alternates sweeps and the likelihood feeding the forward message absorbs
backward information from the second iteration onward. Documented, with a
pointer to filtered_log_evidence for the quantity that survives convergence.
That trap is one function away from the one #19 was about.

#23 — color_greedy carried #[allow(dead_code)] despite being called by
recompute_color_groups: a mute button on a live function, which is the
specific complaint in that issue.

#27 was already fixed — the guard landed in f4e2922 and the issue was filed
against 7742b2b, which merge-base confirms predates it — but nothing pinned
it. Added the issue's own reproduction, which matters because the two profiles
fail differently and a debug-only test would miss the release path. Removing
both guards reproduces the issue verbatim: "attempt to subtract with overflow"
in debug, "index out of bounds: the len is 0 but the index is
18446744073709551615" in release.

Also amended the filtered-estimates spec (#30): the tolerance-not-bit-identity
caveat is conservative. Forcing the scratch onto the sequential sweep instead
of the grouped one — a far larger perturbation than a permuted event order —
still agrees within 1e-8 under tight convergence.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T5SYDExxL4vZgvunrcNSMc

src/time_slice.rs

@@ -305,8 +305,9 @@ impl<T: Time> TimeSlice<T> {
                     *idx,
                     Skill {
                         forward: agents[*idx].receive(&self.time),
+                        backward: N_INF,
+                        likelihood: N_INF,
                         elapsed,
-                        ..Default::default()
                     },
                 );
             }