feat(fabric): let a subscriber callback fail so the event is redelivered (RIG-4030)

[rigel-mintaka]

This PR is part of a stack containing 6 PRs:

1. main
2. feat(comms): publish message_posted on the event fabric (RIG-3107) #1333
3. feat(fabric): propagate trace context, add OnReconnect, pin serial callbacks (RIG-3107) #1337
4. feat(delivery): consume message_posted from the event fabric (RIG-3107) #1345
5. fix(delivery): fire a hold that lost the race to its settle; skip held in start sweeps (RIG-4031) #1351
6. test(delivery): prove single-claim across instances and slow-callback redelivery (RIG-3107) #1353
7. "feat(fabric): let a subscriber callback fail so the event is redelivered (RIG-4030)" (this PR)

The EventFabric callback now returns an error. nil acks; an error or a
panic Naks through retryOrPark and parks on DLQSubject at MaxDeliver.

Delivery returns an error for a transient read before any hold or
dispatch, and nil for a missing row or a deliberate skip, so a retry
starts clean. Reads inside fanOut still log and ack, since a retry would
repeat mention steers. hold keeps held entries in commit order, so a
redelivered ref cannot fire behind a later one.

Also proves the cutover record T4 (b)/(c): error redelivery to MaxDeliver
and the DLQ park with Compass-Original-Subject.

Built on the recommended option; draft until that decision is ruled.

Spec-impact: pending RIG-4030 (the callback signature in the cutover
record T2 Interfaces line). Refs RIG-4030, RIG-3107

Co-authored-by: Matt Wilkinson matt@rigel.build

[linear-code]

RIG-4030

RIG-3107

[github-actions]

Compass engineering docs preview: https://compass-managed-rig-4030-cal.compass-eng-docs.pages.dev

Deployed from compass-managed/rig-4030-callback-error at 5b5c8cb.

Changed pages:

• go/internal/fabric/SUBJECTS.md