feat(http): implement /healthz raw route and stealth decoy fallback

GET /healthz: raw route, no auth, no OperationContext, returns 200 OK
with plain-text 'ok' (ADR-036). Decoy fallback for unknown paths via
DecoyConfig: fake nginx 404 (default), static site serving, or redirect.
Decoy does not leak alknet presence (no alknet headers/format). Custom
routes take precedence over decoy (decoy is fallback only). Wire real
handlers into HttpAdapter router replacing placeholder 501s.

crates/alknet-http/src/server/mod.rs

@@ -7,5 +7,9 @@
 //! `docs/architecture/crates/http/http-server.md`.
 
 pub mod adapter;
+pub mod decoy;
+pub mod healthz;
 
-pub use adapter::{DecoyConfig, HttpAdapter};
+pub use adapter::{DecoyConfig, HttpAdapter};
+pub use decoy::decoy_fallback;
+pub use healthz::healthz;