feat(beliefs): temporal scope and validity; state changes supersede rather than contradict

Beliefs gain a temporal scope (persistent, transient or unknown, classified
deterministically from the proposition), an observation time and, for
transient states, a validity interval. When a newer belief conflicts with
an older transient one observed at least a minute earlier, the older belief
is closed (valid_until set, status superseded, a supersedes link, a version
and an event saying 'state changed') with its score untouched: 'the door
was open at 10:00' stays true of 10:00 when it is closed at 10:05.
Simultaneous conflicting reports and persistent propositions contradict as
before. Introspection lists superseded states with their validity.
Migration 0020 is additive.

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
+- 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 A later observation that a transient state changed closes the earlier belief (valid_until, superseded, score unchanged) instead of contesting it; simultaneous conflicts and persistent propositions still contradict.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.
 - Confidence semantics: every breakdown states that the number is an uncalibrated epistemic score; belief detail adds a calibrated estimate from the mind's resolved predictions when at least twenty were decided in the same range (otherwise says why not) and names the primary uncertainty deterministically. `docs/beliefs.md` defines every epistemic number.
 - Explicit `unknown` belief status: no live evidence either way is neither tentative belief nor falsity. Migration 0019.
 - Assertions: every statement made to a mind is recorded first as an append-only assertion (attributed actor, stance, certainty, modality, negation, reported or quoted speech, hypothetical status, interpretation confidence and notes) before, and separately from, any belief about the proposition. Reported speech is attributed to the person reported and weighed as second-hand; hypotheticals, questions and imperatives create no belief; expressed doubt counts against. API `GET /api/v1/minds/{mind}/assertions`, belief pages show who said so. Migration 0018.