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.

migrations/0018_assertions.down.sql

@@ -0,0 +1 @@
+DROP TABLE IF EXISTS assertions;

migrations/0018_assertions.up.sql

@@ -0,0 +1,34 @@
+-- Assertions (review pass §7.2 item 1): the recorded fact that an actor
+-- stated a proposition, kept apart from any belief about the proposition.
+CREATE TABLE assertions (
+  id                        uuid PRIMARY KEY,
+  mind_id                   uuid NOT NULL REFERENCES minds(id),
+  actor                     text NOT NULL,             -- who spoke to the mind
+  actor_kind                text NOT NULL DEFAULT 'user',
+  attributed_to             text NOT NULL,             -- whose stance this is (speaker, or the person they report)
+  proposition               text NOT NULL,
+  subject                   text NOT NULL DEFAULT '',
+  stance                    text NOT NULL CHECK (stance IN ('asserts','denies','doubts','questions','supposes','reports')),
+  certainty                 text NOT NULL CHECK (certainty IN ('asserted','hedged','uncertain','emphatic')),
+  modality                  text NOT NULL CHECK (modality IN ('factual','hypothetical','reported','quoted','imperative','question')),
+  negated                   boolean NOT NULL DEFAULT false,
+  reported                  boolean NOT NULL DEFAULT false,
+  reported_actor            text NOT NULL DEFAULT '',
+  quoted                    boolean NOT NULL DEFAULT false,
+  hypothetical              boolean NOT NULL DEFAULT false,
+  interpretation_confidence double precision NOT NULL CHECK (interpretation_confidence >= 0 AND interpretation_confidence <= 1),
+  interpretation_notes      text[] NOT NULL DEFAULT '{}',
+  source_event              uuid,
+  conversation_id           uuid,
+  message_id                uuid,
+  context                   text NOT NULL DEFAULT '',
+  trust                     text NOT NULL DEFAULT 'user',
+  belief_id                 uuid,                      -- the proposition belief this fed, if any
+  created_at                timestamptz NOT NULL DEFAULT now()
+);
+CREATE INDEX assertions_mind_time_idx ON assertions (mind_id, created_at DESC);
+CREATE INDEX assertions_mind_actor_idx ON assertions (mind_id, attributed_to);
+CREATE INDEX assertions_belief_idx ON assertions (belief_id) WHERE belief_id IS NOT NULL;
+CREATE INDEX assertions_source_idx ON assertions (source_event) WHERE source_event IS NOT NULL;
+CREATE TRIGGER assertions_append_only BEFORE UPDATE OR DELETE ON assertions
+  FOR EACH ROW EXECUTE FUNCTION noema_forbid_change();