feat(llm): Amazon Bedrock provider over a standard-library SigV4 signer

Kind `bedrock` speaks the Converse API for chat roles and InvokeModel for
Titan and Cohere embeddings. Requests are signed with AWS Signature
Version 4 implemented with crypto/hmac and crypto/sha256 only; the signer
is pinned by tests to signatures botocore produces for the same requests,
with and without a session token, so drift fails the build.

The credential is one secret reference, ACCESS_KEY_ID:SECRET_ACCESS_KEY
[:SESSION_TOKEN], encrypted together like every other provider key. The
region is read from the endpoint host, which must be a Bedrock runtime
host over HTTPS unless private endpoints are explicitly allowed; other
hosts are refused. Provider errors are scrubbed of the secret and token.

Migration 0024 widens the provider kind constraint; existing rows are
untouched and the down migration refuses while bedrock rows exist rather
than dropping configuration. UI kind selects, docs/llm-providers.md,
ADR-0010, ARCHITECTURE and the changelog are updated.

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
+- Amazon Bedrock provider: Converse API for chat roles and Titan/Cohere embeddings, signed with a standard-library SigV4 implementation pinned to botocore reference signatures; credentials as one encrypted reference; non-Bedrock endpoints refused.
 - Privacy redaction (ADR-0009): the audited path that makes an event, memory, message, model call, belief or assertion irrecoverable while keeping the row, its links, timestamps and scores; a ledger with a hash of the removed content; cascade to derived rows and the events that quote them; stored snapshot archives scrubbed and re-signed; replay labels redacted stimuli. `noema_redact()` is the only write the append-only trigger admits. Migration 0023. `docs/privacy.md` defines retained, redacted, irrecoverable and deleted.
 - Causal timeline: `GET /api/v1/events/{id}/chain` and `/events/{id}/timeline` walk the links events actually record (causation, parents, cited evidence) backwards and forwards within bounded depth and size, label edge kinds, group by cycle as correlation only, and infer nothing.
 - 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).