fix(mat): gate api module behind its feature — standalone no-default-features builds

pub mod api was unconditional while its only dependency, serde, is optional
behind the 'api' feature, so any build without default features failed with
101 unresolved-serde errors (masked in --workspace runs by feature
unification). The api module and its create_router/AppState re-export are now
cfg(feature = "api")-gated with docsrs annotations.

All combos compile: bare --no-default-features (was 101 errors, now 0),
--no-default-features --features api, and full default (177 tests pass).
Workspace gate: 2,918 passed / 0 failed.

https://claude.ai/code/session_01MjBucx95K4BuUxZi8NWwRH

v2/crates/wifi-densepose-mat/src/lib.rs

@@ -78,6 +78,10 @@
 #![warn(rustdoc::missing_crate_level_docs)]
 
 pub mod alerting;
+/// REST API surface (Axum). Requires the `api` feature — its DTOs derive
+/// serde, which is an optional dependency gated behind that feature.
+#[cfg(feature = "api")]
+#[cfg_attr(docsrs, doc(cfg(feature = "api")))]
 pub mod api;
 pub mod detection;
 pub mod domain;
@@ -122,6 +126,8 @@ pub use integration::{
     AdapterError, HardwareAdapter, IntegrationConfig, NeuralAdapter, SignalAdapter,
 };
 
+#[cfg(feature = "api")]
+#[cfg_attr(docsrs, doc(cfg(feature = "api")))]
 pub use api::{create_router, AppState};
 
 pub use ml::{