feat(experiments): cognitive forks per model with per-mind routing and comparison

A mind's configuration may override model routing per role (llm_roles);
the router honours it for that mind only and fails closed when the named
model is missing or disabled, so a comparison between providers can never
silently fall back to the default. Experiments may declare variants: each
becomes its own fork of the same snapshot with its own configuration,
personality or routing, receives identical stimuli, and is measured. The
results carry one entry per variant and a deterministic comparison: first
divergent stimulus, belief and attention overlap, mean score difference on
shared propositions, and the ranges of memory counts, goal counts, cost and
capability requests. Single-subject definitions are unchanged.

CHANGELOG.md

@@ -10,6 +10,7 @@ All notable changes to Noema are recorded here. Format follows Keep a Changelog;
 - Security suite: SQL-injection effect test, static allow-list scan for assembled SQL, deceived-mind authority test, garbage model output test, self-preservation vocabulary test; threat model rewritten so every verification cites a test that asserts the claim.
 
 ### Added
+- Cognitive forks per model: minds may override model routing per role (`llm_roles`, fail-closed); experiments may declare variants that fork one snapshot under different configuration, personality or routing, receive identical stimuli, and are compared deterministically (first divergent stimulus, belief and attention overlap, score differences, memory, goal, cost and capability-request ranges).
 - Runtime identity: snapshots, replays and experiments record build (commit, modified tree, Go version, platform), schema version, a secret-free configuration digest, personality version, policy digest, model routing, prompt versions, deterministic subsystem versions and timezone. Replays compare identities and label themselves exact-candidate, different-build or incompatible (the last is never run), and analyse the first divergence by component. Migration 0022.
 - Contextual source reliability: a per-(source, domain) record updated only by prediction outcomes and operator corrections, adjusting evidence credibility in that domain once three outcomes exist; never global. Shown on belief pages. Migration 0021.
 - Temporal beliefs: every belief has a temporal scope (persistent, transient, unknown, classified deterministically), an observation time and, for transient states, a validity interval. A later observation that a transient state changed closes the earlier belief (valid_until, superseded, score unchanged) instead of contesting it; simultaneous conflicts and persistent propositions still contradict. Introspection reports what was true and when. Migration 0020.