A group declares a declaratively-authored [[triggers.dead_letter]] shared=true
handler; when a message in the group's SHARED queue is exhausted, the
dispatcher's q_terminal group branch (after persisting to group_dead_letters)
fans out to it via list_matching_shared_dead_letter(owning_group, "queue", …),
each outbox row stamped the WRITER app_id (the consuming app — the M2 shared
-write model), so the handler runs under the consumer. The per-app
list_matching_dead_letter gained AND t.shared = FALSE (the shared flag is the
namespace boundary); the owning-group filter is the isolation boundary.
Adds BundleTrigger::DeadLetter + a DeadLetterTriggerSpec manifest kind (group
+shared only — validate_bundle_for rejects app-owned or non-shared, and exempts
it from the shared-requires-a-collection rule); insert_trigger_tx now accepts
dead_letter and writes dead_letter_trigger_details; current_trigger_identity
matches a group-shared dead_letter so re-apply is a NoOp (app-owned ones stay
diff-invisible). Pinned by tests/shared_dead_letter.rs (owning group matches,
per-app query does not, foreign group does not).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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