Add canonical event log and renderer skeleton (A10)

Build-order step 4, sequenced before the LLM client (step 3) because
the client interface consumes the canonical log.

- canonical.py: typed events for the full A10 table (user_message
  through compaction); canonical tool-call IDs minted locally,
  provider IDs never a join key; strict payload validation on load
- StateStore: canonical_events table via schema v2 — the first real
  migration, exercised in a test against a hand-built v1 database —
  with append_event/read_events on the same fsync-per-commit path
- render.py: request bodies rendered from the log at call time;
  first adapter targets the OpenAI-compatible shape of LiteLLM's
  normalized route; compaction events replace their id range at
  render time while staying in the durable log
- Crash integration test now also verifies transcript appends
  survive process death

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/integration/test_crash_durability.py

@@ -15,11 +15,13 @@ pytestmark = pytest.mark.integration
 CRASH_SCRIPT = """
 import os
 import sys
+from nisse_dev.canonical import UserMessage
 from nisse_dev.state import StateStore
 
 store = StateStore(sys.argv[1])
 store.transition("EXECUTING", cause="test", project="demo", data={"milestone": 2})
 store.begin_intent("push_branch", {"branch": "milestone-2"}, cause="test")
+store.append_event(UserMessage(text="start milestone 2", source="m:1"))
 os._exit(1)  # die without closing anything, as a power cut would
 """
 
@@ -45,3 +47,6 @@ def test_state_survives_process_death(tmp_path: Path) -> None:
         assert report.action == "await_input"
         [pending] = store.pending_intents()
         assert pending.payload == {"branch": "milestone-2"}
+
+        [stored] = store.read_events()
+        assert stored.event.text == "start milestone 2"