feat: authored-history-write consumer side (Worldtree #347) — v0.19.6

Consumer side of Worldtree's #347 authored-history-write (the SillyTavern
first-message primitive), shipped via direct in-session TDD:

- write_authored_history (POST /sessions/{id}/history): v1 author=assistant,
  effects=none, per-session idempotency; body server-pinned (AuthoredWriteRequest
  extra=forbid) so null effects/claimed_original_at are omitted; 200 replay /
  201 fresh both return the AuthoredTurnResponse dict.
- AuthoredHistoryUnavailable: the hide-existence 404 (feature-absent / ungranted
  / session-absent, indistinguishable by design — INV-347-1) raised DISTINCT from
  SessionApiFailed so callers branch feature-absent and never capability-probe.
- get_session_messages (GET /sessions/{id}/messages): un-deferred as the seed
  read-back — confirms a seed renders as a normal role=assistant turn
  (model-invisible provenance).
- --seed-first-message probe: create session -> seed -> read-back; a 404 reports
  a benign feature-absent result (exit 0), never a capability-probe.

Contract #2 amended (2 FNs, validated OK). 19 new tests (12 wrapper + 7 cli),
suite 601 green. Coverage-map re-converged: REST 19/41 (the #347 route + the
messages read-back close the one gap the 2.3.0 re-vendor opened).

Live-proof pending the session.history.write grant (requested infra-ops).

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "ratatoskr"
-version = "0.19.5"
+version = "0.19.6"
 description = "Worldtree Conversation API debug TUI — multi-pane observability dashboard"
 readme = "README.md"
 requires-python = ">=3.12"