runtime: prove the host is venue-agnostic (zero-leak gate)

[mfw78]

Prove the host is venue-agnostic by landing a permanent zero-leak CI check and making it a blocking, required gate.

Goal

Enforce, forever, that the host layer never regains intent, venue or cow knowledge. The check greps the runtime crate for forbidden symbols and inspects its dependency graph for forbidden crate edges, and an echo-venue boot test acts as the oracle that the generic seam actually works.

Scope

This epic adds a permanent zero-leak and acyclicity CI check over nexum-runtime, then flips it from advisory to blocking once the HostState.pool_router field is deleted and the router is carried only through the generic services map. The forcing function is that an echo-venue must still install and route a submission with no intent or cow crate anywhere in the graph, which demonstrates the host is genuinely venue-agnostic rather than merely refactored.

Milestone: M2: Generic venue-agnostic host.

[mfw78]

Epic complete. Both sub-issues landed (squash-merged into dev/m1):

• engine: add the zero-leak CI check for the host layer #384 (add the zero-leak CI check) via ci: align the zero-leak check to the charter symbol set #449
• runtime: prove the host is venue-agnostic (zero-leak CI blocking) #385 (prove venue-agnostic, flip to blocking) via ci: block the zero-leak gate on router-field and wit-namespace scans #450

Scope delivered: a permanent symbol + crate-graph zero-leak scan over nexum-runtime, now a blocking gate, with the echo-venue boot oracle (crates/videre-host/tests/zero_leak.rs) as the functional proof, and the HostState.pool_router precondition met (deleted earlier in #443; 0 occurrences in nexum-runtime/src).

Note: during review the gate's symbol scans were corrected to the live post-rename vocabulary (Venue[A-Z] / VenueRegistry, not the retired nexum:intent/PoolRouter), so the check genuinely catches a real leak rather than merely running - bite-tested that a reintroduced videre_host::VenueRegistry field in nexum-runtime/src now fails the gate.

Closing manually since the merges targeted dev/m1 (not the default branch).