feat(epistemics): record assertions between observations and beliefs

A statement made to a mind is now three things kept apart: the observation
that it arrived, an append-only assertion (attributed actor, stance,
certainty, modality, negation, reported or quoted speech, hypothetical
status, interpretation confidence and notes), and, only for factual
assertions, one piece of evidence for a proposition belief attributed to
whoever actually stands behind it. Reported speech is attributed to the
person reported and weighed as second-hand; hypotheticals, questions and
imperatives create no belief; expressed doubt counts against; a possibly
sarcastic remark is recorded with low interpretation confidence rather
than guessed at. Interpretation is deterministic code; a model may enrich
the claim list but never sets stance or confidence.

Association no longer treats conversational turns as world events:
hypotheticals, questions, requests and consecutive turns by one speaker
cannot seed causal hypotheses.

Adds internal/epistemics, migration 0018, the assertions API, assertions
on belief pages and in belief detail, a shared end-to-end test harness
(tests/harness), end-to-end tests for eight speech patterns, and updates
the milestone test to the new provenance.

docs/beliefs.md

@@ -37,6 +37,10 @@ confidence    = sigmoid( logit(prior) + 1.5 × (for × diversity − against) )
 
 Every evidence row records its `derivation`, `family`, `root_ids` (the originating events or memories) and a `provenance_status`. Rows written before provenance existed are marked `legacy_unknown` with `family = source`; no ancestry is invented for them. The breakdown (prior, per-family contributions, for, against, families, diversity, log-odds, formula, notes) is stored with every version and shown on the belief page under "Independent sources".
 
+## Assertions as evidence
+
+Statements people make reach beliefs only through assertions (`docs/conversation.md`). An `assertion` evidence row carries the attributed actor as its family, the source message as its root, and a weight that already reflects hedging, reporting and interpretation confidence. Nothing is silently upgraded: a claim stays a claim until evidence from another family arrives.
+
 ## Status
 
 Derived from the breakdown unless an operator pinned it: