feat: qualify dedup IDs and bound the exporter queue

[savme]

The exporter's informer callback called NATS publish directly, so a slow or unreachable NATS connection blocked the callback outright. Adds a bounded queue between the two: the callback enqueues without blocking, one goroutine drains it into NATS, and a full queue drops the newest event and counts it rather than blocking.

This also introduces qualifiedMsgID, so that messages are deduped cross-cluster correctly.

Test plan

• go test
• Covers drop-newest-on-full-queue behavior directly, including that an already-queued event is kept over the incoming one

Depends on #246

Stack:

• fix: shared processor event extractors #239
• feat: add federated events schema #240
• feat: add federated event Source/Resource/Origin support #245
• feat: add federated events exporter #246
• feat: qualify dedup IDs and bound the exporter queue #248 👈
• feat: federated events transport #250