WP-0001-T011: append-only event log — write, fetch_since, tail, replay

src/artifactstore/events/:
- model.py: Event frozen dataclass (event_type, subject_kind, subject_id,
  actor, payload, payload_digest; sequence + created_at populated by the
  DB on write). make_event() helper computes payload_digest as raw BLAKE3
  (32 bytes) of payload. ViewWriter Protocol with reset() + apply().
- log.py:
  * write(connection, event) — inserts one row in the caller's transaction
    and returns Event with sequence + created_at populated via RETURNING.
  * fetch_since(connection, since_sequence, limit) — read events after a
    cursor in order.
  * tail(engine, since_sequence) — async-iterator long-poll over the log;
    SQLite uses interval polling, PG LISTEN/NOTIFY is a future workplan.
  * replay(engine, view_writer, reset=True) — drains the event log through
    a ViewWriter inside one transaction; returns the highest sequence
    applied.
- views.py: RegistryViewWriter — canonical event handlers shared by direct
  write and replay paths. Ships handlers for v1.package.created (inserts
  artifact_packages + retention_state) and v1.package.finalized (updates
  status, finalized_at, manifest_digest). Unknown event types tolerated;
  additional handlers register here as later tasks land.

src/artifactstore/db/schema.py: events.sequence type is now
BigInteger().with_variant(Integer(), 'sqlite') so SQLite's autoincrement
(INTEGER PRIMARY KEY rowid alias) works while PostgreSQL keeps BIGSERIAL.

tests/integration/test_event_log.py (6 cases):
- write() assigns monotonic sequence numbers (1, 2, ...) and a created_at.
- fetch_since(since_sequence=2) returns the ordered tail.
- tail() yields events and exits cleanly on consumer break.
- Direct write path (write + apply) and replay path produce byte-identical
  materialised state — the key ADR-0002 invariant.
- Replay handles multiple event types (package.created -> finalized).
- Unknown event types are tolerated (no-op apply).
- payload_digest equals BLAKE3 of payload.

Gates: ruff clean, mypy --strict clean on 36 files, 45 tests pass.
make migrate-fresh end-to-end ok.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/integration/test_event_log.py

@@ -0,0 +1,236 @@
+"""Event log integration tests (ARTIFACT-STORE-WP-0001-T011)."""
+
+from __future__ import annotations
+
+import asyncio
+import uuid
+from collections.abc import AsyncIterator
+from pathlib import Path
+
+import cbor2
+import pytest
+import pytest_asyncio
+from sqlalchemy import insert, select
+from sqlalchemy.ext.asyncio import AsyncEngine, create_async_engine
+
+from artifactstore.db.schema import (
+    artifact_packages,
+    metadata,
+    retention_classes,
+    retention_state,
+)
+from artifactstore.db.seed import RETENTION_CLASS_SEEDS
+from artifactstore.events import (
+    RegistryViewWriter,
+    fetch_since,
+    make_event,
+    replay,
+    tail,
+    write,
+)
+
+
+@pytest_asyncio.fixture
+async def fresh_engine(tmp_path: Path) -> AsyncIterator[AsyncEngine]:
+    db_path = tmp_path / "events.db"
+    engine = create_async_engine(f"sqlite+aiosqlite:///{db_path}")
+    async with engine.begin() as conn:
+        await conn.run_sync(metadata.create_all)
+        for seed in RETENTION_CLASS_SEEDS:
+            await conn.execute(insert(retention_classes).values(**seed))
+    yield engine
+    await engine.dispose()
+
+
+def _package_created_payload(*, name: str = "p", retention_class: str = "raw-evidence") -> bytes:
+    return cbor2.dumps(
+        {
+            "name": name,
+            "producer": "test-producer",
+            "subject": "test-subject",
+            "retention_class": retention_class,
+            "metadata": {"k": "v"},
+        },
+        canonical=True,
+    )
+
+
+async def test_write_assigns_monotonic_sequence(fresh_engine: AsyncEngine) -> None:
+    e1 = make_event(
+        event_type="v1.system.note",
+        subject_kind="system",
+        actor="t",
+        payload=b"hi",
+    )
+    e2 = make_event(
+        event_type="v1.system.note",
+        subject_kind="system",
+        actor="t",
+        payload=b"hello",
+    )
+    async with fresh_engine.begin() as conn:
+        r1 = await write(conn, e1)
+        r2 = await write(conn, e2)
+    assert r1.sequence == 1
+    assert r2.sequence == 2
+    assert r1.created_at is not None
+    assert r2.created_at is not None
+
+
+async def test_fetch_since_returns_ordered_subset(fresh_engine: AsyncEngine) -> None:
+    async with fresh_engine.begin() as conn:
+        for i in range(5):
+            await write(
+                conn,
+                make_event(
+                    event_type="v1.system.note",
+                    subject_kind="system",
+                    actor="t",
+                    payload=f"n{i}".encode(),
+                ),
+            )
+    async with fresh_engine.connect() as conn:
+        batch = await fetch_since(conn, since_sequence=2)
+    seqs = [e.sequence for e in batch]
+    assert seqs == [3, 4, 5]
+
+
+async def test_tail_yields_events_then_breaks(fresh_engine: AsyncEngine) -> None:
+    pkg_id = uuid.uuid4()
+    async with fresh_engine.begin() as conn:
+        await write(
+            conn,
+            make_event(
+                event_type="v1.package.created",
+                subject_kind="package",
+                subject_id=pkg_id,
+                actor="ops",
+                payload=_package_created_payload(),
+            ),
+        )
+
+    collected = []
+
+    async def consume() -> None:
+        async for evt in tail(fresh_engine, since_sequence=0, poll_interval_seconds=0.01):
+            collected.append(evt)
+            if len(collected) >= 1:
+                break
+
+    await asyncio.wait_for(consume(), timeout=3.0)
+    assert len(collected) == 1
+    assert collected[0].event_type == "v1.package.created"
+    assert collected[0].subject_id == pkg_id
+
+
+async def test_replay_reproduces_direct_write_state(fresh_engine: AsyncEngine) -> None:
+    """Direct path (write + apply) and replay path produce identical state."""
+    writer = RegistryViewWriter()
+    pkg_id = uuid.uuid4()
+
+    # Direct path: write event + apply in one transaction.
+    async with fresh_engine.begin() as conn:
+        evt = await write(
+            conn,
+            make_event(
+                event_type="v1.package.created",
+                subject_kind="package",
+                subject_id=pkg_id,
+                actor="ops",
+                payload=_package_created_payload(),
+            ),
+        )
+        await writer.apply(conn, evt)
+
+    async with fresh_engine.connect() as conn:
+        pkg_a = (await conn.execute(select(artifact_packages))).one()
+        ret_a = (await conn.execute(select(retention_state))).one()
+
+    # Replay path: reset views and replay every event.
+    applied_seq = await replay(fresh_engine, writer, reset=True)
+    assert applied_seq == evt.sequence
+
+    async with fresh_engine.connect() as conn:
+        pkg_b = (await conn.execute(select(artifact_packages))).one()
+        ret_b = (await conn.execute(select(retention_state))).one()
+
+    # Materialised state must match across direct vs replay paths.
+    assert tuple(pkg_a) == tuple(pkg_b)
+    assert tuple(ret_a) == tuple(ret_b)
+
+
+async def test_replay_handles_multiple_event_types(fresh_engine: AsyncEngine) -> None:
+    writer = RegistryViewWriter()
+    pkg_id = uuid.uuid4()
+
+    async with fresh_engine.begin() as conn:
+        e_created = await write(
+            conn,
+            make_event(
+                event_type="v1.package.created",
+                subject_kind="package",
+                subject_id=pkg_id,
+                actor="ops",
+                payload=_package_created_payload(),
+            ),
+        )
+        await writer.apply(conn, e_created)
+
+        e_finalized = await write(
+            conn,
+            make_event(
+                event_type="v1.package.finalized",
+                subject_kind="package",
+                subject_id=pkg_id,
+                actor="ops",
+                payload=cbor2.dumps(
+                    {"manifest_digest_hex": "ab" * 32},
+                    canonical=True,
+                ),
+            ),
+        )
+        await writer.apply(conn, e_finalized)
+
+    async with fresh_engine.connect() as conn:
+        pkg_a = (await conn.execute(select(artifact_packages))).one()
+    assert pkg_a.status == "finalized"
+    assert pkg_a.manifest_digest == bytes.fromhex("ab" * 32)
+
+    last_seq = await replay(fresh_engine, writer, reset=True)
+    assert last_seq == e_finalized.sequence
+
+    async with fresh_engine.connect() as conn:
+        pkg_b = (await conn.execute(select(artifact_packages))).one()
+    assert tuple(pkg_a) == tuple(pkg_b)
+
+
+async def test_unknown_event_type_is_tolerated(fresh_engine: AsyncEngine) -> None:
+    writer = RegistryViewWriter()
+    async with fresh_engine.begin() as conn:
+        evt = await write(
+            conn,
+            make_event(
+                event_type="v1.unknown.frob",
+                subject_kind="system",
+                actor="ops",
+                payload=cbor2.dumps({}, canonical=True),
+            ),
+        )
+        await writer.apply(conn, evt)  # must not raise
+
+
+async def test_event_payload_digest_is_blake3_of_payload() -> None:
+    import blake3 as _blake3
+
+    payload = cbor2.dumps({"hello": "world"}, canonical=True)
+    evt = make_event(
+        event_type="v1.system.note",
+        subject_kind="system",
+        actor="t",
+        payload=payload,
+    )
+    assert evt.payload_digest == _blake3.blake3(payload).digest()
+
+
+# Acknowledge pytest fixture as used.
+_ = pytest