Add Airbyte protocol v2 compliance (stream status, per-stream state)

[Smidge]

Problem

The connector does not work with Airbyte 2.x. Every sync fails with one of two errors:

1. Missing stream status messages:

   Input was fully read, but some streams did not receive a terminal stream status message.
   

2. LEGACY state rejection (on incremental syncs):

   java.lang.IllegalArgumentException: LEGACY states are deprecated.
   

These were optional in Airbyte 1.x but are now enforced in 2.x (protocol v0.2.0+).

Root Cause

Three issues in the connector:

1. No stream status trace messages. Airbyte 2.x requires STARTED and COMPLETE/INCOMPLETE status messages for every stream. The connector emits none.

2. Legacy global state format. The connector emits a single global state blob ({"data": {"streams": {...}}}). Airbyte 2.x requires per-stream state messages with {"type": "STREAM", "stream": {"stream_descriptor": {...}, "stream_state": {...}}}.

3. Cannot parse v2 state input. On incremental syncs, Airbyte 2.x passes state back as a JSON array of per-stream state objects. The connector only understands the legacy global format, so json.Unmarshal fails and the process exits before reading any data.

Changes

Commit 1: Add stream status trace messages

• Add TRACE message type and STREAM_STATUS constants (STARTED, COMPLETE, INCOMPLETE)
• Add StreamDescriptor, AirbyteStreamStatus, AirbyteTraceMessage types
• Replace State(SyncState) with StreamState(namespace, streamName, ShardStates) that emits per-stream state with type=STREAM
• Add StreamStatus() method to emit stream lifecycle trace messages
• Update AirbyteLogger interface and test mock

Commit 2: Update read loop and state parsing

• Emit STARTED before reading each stream, COMPLETE on success, INCOMPLETE on error
• Replace os.Exit(1) with break on per-stream errors so remaining streams still get status messages
• Emit per-stream STATE after each stream completes (not one global blob at the end)
• Parse Airbyte v2 per-stream state array on incremental syncs, with fallback to legacy format
• Default empty namespace to source database name to prevent state key mismatches

Commit 3: Tests

• 8 logger unit tests (per-stream state format, no legacy fields, stream status traces, JSON round-trip)
• 6 read protocol integration tests (message ordering, multi-shard state, error handling, per-stream state emission)

Testing

• All existing tests pass
• New tests cover the protocol changes
• Tested against Airbyte OSS v2.0.2 with 28 PlanetScale streams — both initial (full refresh) and incremental syncs succeed
• Verified output with docker run ... read | grep STATE shows "type":"STREAM" per-stream state
• Verified STARTED and COMPLETE trace messages appear for each stream

Compatibility

• Backwards compatible with the legacy state format (falls back automatically)
• No changes to the connector spec, check, or discover commands
• No changes to the Dockerfile or CI configuration