test: pin unknown-venue path when registry service absent

[mfw78]

What

Add one integration test to crates/videre-host/tests/platform.rs pinning the service-lookup-miss UnknownVenue path in client.rs, where services.get::<VenueRegistry>(NAMESPACE) returns None.

A minimal ClientWithoutRegistry(Videre) Extension wrapper forwards every face boot_single consults (namespace, capabilities, link, manifest_sections, subscriptions, admit_worker) to the real Videre, but leaves service() at its None default, so HostServices::from_extensions seeds no venue registry.

The test client_without_registry_service_resolves_every_venue_to_unknown boots the prebuilt echo-client keeper via Supervisor::boot_single with a [venue]-free manifest, asserts supervisor.services().get::<VenueRegistry>(NAMESPACE).is_none() as the branch precondition, dispatches a chain-1 block, and asserts the keeper stays alive with quote and submit each resolving to unknown-venue.

No production code is touched.

Why

The service-missing branch (registry service absent entirely) is distinct from the already-covered adapter-map miss (registry present, venue id merely unlisted). This test pins the former end to end, exercising the resolve-to-unknown path before any adapter is consulted.

A [venue]-free manifest is required because a keeper declaring [venue] would be refused by admit_worker before it could reach the client face, given no adapters boot under boot_single.

Testing

cargo fmt --all -- --check passes with no diff.

cargo clippy -p videre-host --all-targets --all-features clean after cargo clean -p videre-host.

The test has genuine teeth: seeding a real registry flips the is_none() precondition and fails the test. CI builds the echo-client wasm before nextest, so the test runs rather than skips.

AI Assistance

Implemented, reviewed and verified with Claude Code (Opus).

Closes #510