Compliance suite: dead letters and the projection error path

[jeremydmiller]

Part of #5118 (compliance wave growth), epic #5110.

AsyncDaemonCompliance is deliberately a smoke test: one async snapshot, one rebuild. The error path — a projection that throws, the event being dead-lettered, the shard continuing — is the behavior that actually costs users when it drifts, and it is declared on shared interfaces:

• JasperFx.Events.Daemon.DeadLetterEvent, DeadLetterShardCount
• IEventDatabase.CountDeadLetterEventsAsync(ShardName), QueryDeadLetterEventsAsync(ShardName, tenantId), FetchDeadLetterCountsAsync() and its per-tenant overload
• IEventStore.AllDatabases() → IReadOnlyList<IEventDatabase>
• IEventFailureContext

Seam additions required

Small. AllDatabases() is already on the shared IEventStore the fixture exposes, so the dead-letter queries are reachable today. What is missing is a way for a suite to register a projection that fails deterministically and to configure the error policy:

1. ComplianceStoreConfig needs an error-handling knob — at minimum "skip poison events after N attempts and record a dead letter", which both products spell differently (Marten OnException(...).SkipEvent() policies on DaemonSettings; Polecat's equivalent). Follows the MaxConcurrentRebuildsPerDatabase precedent: fixture-owned property, not registrar-routed.
2. Nothing else — the failing projection itself is shared source.

What each store tests today

Marten	Polecat
DaemonTests error-handling and skip tests	Daemon/dead_letter_count_tests.cs
marking_events_as_skipped_as_Guid_identified.cs	Daemon/skipped_events_count_augmenter_tests.cs
marking_events_as_skipped_as_string_identified.cs	Daemon/shard_failure_progression_columns_tests.cs
	Events/event_failure_context_tests.cs

Scope

New DeadLetterCompliance suite:

1. A projection that throws on one event type dead-letters that event and the shard keeps advancing
2. CountDeadLetterEventsAsync / FetchDeadLetterCountsAsync report the shard and the count
3. QueryDeadLetterEventsAsync returns the event body, the exception type and the shard name
4. IEventFailureContext correlates the failure with the dead-letter row
5. A rebuild clears prior dead letters for the shard
6. Marking events as skipped, for Guid and string stream identity

Explicitly out of scope: pause/resume policies, circuit breakers, multi-node failure handling — daemon-topology behavior that stays product-specific.

Acceptance

• One config knob added; suite enrolled in both stores
• Originals retired for the absorbed behavior in both repos