Extract the event log codec used by append and replay

[ThomasK33]

What to build

Deepen event-log parsing and replay validation into a shared event-log codec module. EventLog.open(), replay loading, and export/offline replay readers should rely on one implementation for JSONL parsing, size limits, record validation, and contiguous sequence checks.

Acceptance criteria

• The event-log size limit is defined in one place and used by append/hydration and replay readers.
• JSONL parsing and contiguous sequence validation are shared between src/host/eventLog.ts and src/host/replay.ts.
• Event record shapes and public replay/export behavior remain unchanged.
• Existing event-log unit tests, replay unit tests, and event-log integration tests still pass.

Blocked by

None - can start immediately.

[ThomasK33]

This was generated by AI during triage.

Agent Brief

Category: enhancement
Summary: Extract a shared event-log codec for append/hydration, replay, and export readers.

Current behavior:
The event log has multiple readers that each parse JSONL records, enforce the event-log size limit, validate event record shapes, and check sequence ordering. The live event log hydration path and replay/export reader path duplicate this logic and currently keep the shared 50 MB size limit in sync by convention.

Desired behavior:
Event-log parsing and validation should be centralized in one shared codec module. EventLog.open(), replay loading, offline replay, and record export readers should all use the shared implementation for JSONL parsing, file-size limits, event record schema validation, and contiguous sequence validation. Event record shapes and public replay/export behavior should remain unchanged.

Key interfaces:

• EventLog.open() hydration behavior — should hydrate existing event logs through the shared codec before deriving the next sequence number.
• readEventLogRecords() behavior — should continue returning validated EventRecord[] values for replay/export callers, but should delegate JSONL parsing and sequence checks to the shared codec.
• Event-log size limit constant — should be defined in one shared location and reused by append/hydration and replay/export readers.
• buildReplayInput() event validation — should preserve replay input validation semantics without introducing a second JSONL parsing path.

Acceptance criteria:

• The event-log size limit is defined in one place and used by append/hydration and replay/export readers.
• JSONL parsing is shared between the live event log hydration path and replay/export readers.
• Contiguous sequence validation is shared rather than duplicated across live hydration and replay/export paths.
• Event record shapes and public replay/export behavior remain unchanged.
• Existing event-log unit tests pass.
• Existing replay unit tests pass.
• Existing event-log integration tests pass.

Out of scope:

• Changing the event record schema or event type payloads.
• Changing CLI JSON envelopes, replay/export output formats, or artifact manifest formats.
• Changing the 50 MB size limit value.
• Reworking event buffering, append ordering, or runtime retention behavior beyond what is needed to share the codec.