[jeremydmiller]

Closes #5705.

The bug

AddMartenHighWaterHealthCheck() only works with the default store from AddMarten():

• HighWaterHealthCheck takes IDocumentStore, which isn't registered when an application uses only AddMartenStore<TStore>(). The first probe throws Unable to resolve service for type 'Marten.IDocumentStore'.
• autoRestart only ever resolved the unmarked IProjectionCoordinator, never IProjectionCoordinator<TStore>.

The fix

Two generic overloads, mirroring the existing two (plain filter, and the provider-aware filter from #5061):

Services.AddHealthChecks()
    .AddMartenHighWaterHealthCheck<IInvoicingStore>(staleThreshold: TimeSpan.FromSeconds(30), autoRestart: true);

• Each call registers a check named HighWaterHealthCheck:<store type>, tagged with the store's name. HighWaterHealthCheckExtensions.HighWaterHealthCheckNameFor<TStore>() returns that name.
• The check is the new HighWaterHealthCheck<TStore>, a subclass that resolves TStore and overrides a new protected resolveCoordinator to use IProjectionCoordinator<TStore>.
• Its settings travel with its own registration. The non-generic overloads Replace the HighWaterHealthCheckSettings singleton, so a generic overload doing the same would silently re-configure the default store's check.
• The staleness tracker stays shared. Its keys start with the database identifier, which is the store's StoreName, so two stores never share a key even when they share a database.

Binary compatibility

No existing signature changes. Everything here is new members: two overloads, one helper, one subclass, and a protected virtual. The generic overloads can't steal existing calls, because TStore can't be inferred, so a call has to name it.

Tests

Marten.AspNetCore.Testing/high_water_health_check_for_ancillary_stores.cs. The hosts are built and never started, so a seeded high-water row stays put, which is the stalled agent the check reports.

Test	Pins
a_store_registered_only_through_AddMartenStore_can_be_probed	the issue's repro
the_default_check_still_needs_the_default_store	the store-less overload still means the default store
a_stalled_mark_on_the_ancillary_store_is_reported	detection works on the ancillary store
auto_restart_goes_through_the_ancillary_stores_own_coordinator	a recording IProjectionCoordinator<TStore> is the one asked
each_store_keeps_its_own_settings_and_tracking	two stores in one database, stalled at different marks, each reported on its own
registering_an_ancillary_check_does_not_reconfigure_the_default_one	1h default threshold stays 1h beside a 200ms ancillary one

Two mutations confirm the tests have teeth:

• Replacing the shared settings singleton fails registering_an_ancillary_check_does_not_reconfigure_the_default_one.
• Dropping the coordinator override fails auto_restart_goes_through_the_ancillary_stores_own_coordinator.

These tests live in Marten.AspNetCore.Testing, which CI runs. The existing HighWaterHealthCheckTests live in DaemonTests.ManualOnly, which has no target in build/build.cs and no CI job. They still pass locally (23/23) against this change.

Local, net10.0:

• Marten.AspNetCore.Testing: 122/122.
• markdownlint and cspell are clean on the docs page.

Docs

docs/events/projections/healthchecks.md gains a "Separate stores" section. It also notes that replacing HighWaterHealthCheckSettings only affects the default store's check.

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