fix(types.events): CustomEvent extends BaseEvent so custom-event routing survives the wire

[laithalsaadoon]

Summary

CustomEvent now extends BaseEvent, so it declares the same routing fields every other event declares (id, timestamp, thread_id, thread_name, message_id) and they survive serialization. Before this change LocalWorker._route_event stamped thread_id onto an attribute the model did not declare, which worked in one process but was dropped by EVENT_ADAPTER on the way to the wire. Over a CoordinatorClient, every custom event arrived with thread_id=None: filtered subscriptions (coord.on(cb, thread_id=...)) never fired for custom events, get_events(thread_id) returned them unrouted, and a client-side append_event(CustomEvent(...)) was rejected by the remote coordinator and only logged.

Closes #48.

What changed

• types/events.py: CustomEvent(BaseEvent) with kind and payload; Event becomes Annotated[SystemEvent | CustomEvent, Field(union_mode="left_to_right")]. The discriminated SystemEvent union is tried first and CustomEvent is the fallback for unknown kinds only. Smart-mode union resolution would otherwise tie-break by the number of fields set, and a CustomEvent that now declares every routing field would outscore a fieldless concrete variant such as StartedEvent.
• The serializer emits the declared fields alongside the flattened payload. A payload entry whose key shadows a declared field (id, thread_id, ...) is re-nested under a payload key so it cannot overwrite the event's own routing on a round trip. (codex_plan emits a payload entry named id.)
• network/client.py: CoordinatorClient.append_event stays fire-and-forget and records each failure on a new public append_errors property (last 32, oldest first) at ERROR level, so a caller can observe a rejected append instead of only finding it in a log.
• ai_thread/summarization.py, cli/commands.py, runtime/usage.py, runtime/worker.py: read the now-declared fields directly instead of getattr fallbacks.
• spec/ stubs mirror the new shapes; stubtest-allowlist.txt gains the Event union entry, mirroring the existing SystemEvent one.

Behavior changes

• A top-level key naming a routing field binds to that field: CustomEvent(kind=..., thread_id=x) routes rather than filling payload.
• CustomEvent inherits BaseEvent's frozen config. A caller that mutated an instance in place must use model_copy(update=...).

Wire compatibility

• An old peer parsing a new frame sees id/timestamp/thread_id/thread_name/message_id as unknown keys and sweeps them into payload, exactly as it sweeps any other unknown key today.
• A new peer parsing an old frame gets the routing fields at their defaults (a fresh id, a now() timestamp, thread_id=None), which the runtime gate then stamps as before.

Tests

tests/test_events_custom_wire.py (new) covers three layers: the adapter keeps routing on a stamped custom event and re-nests shadowing payload keys; the runtime gate stamps the declared field; a real CoordinatorEndpoint plus CoordinatorClient agree on routing (filtered subscription fires, get_events returns the id, client-side append_event lands, a rejected append shows up on append_errors). tests/test_pure_functions.py and tests/test_reconstruct_roundtrip.py gain the union-order and round-trip cases.

Gate

Run on this branch (23c9ce39, base 1125f0c4), mcp 2.1.1:

ruff format --check src tests   1 file would be reformatted (pre-existing on main: src/ai_functions/memory/base.py), 108 already formatted
hatch run lint                  All checks passed!
hatch run check-spec            Success: no issues found in 80 modules
hatch run test                  487 passed, 2 skipped