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/Cargo.toml

@@ -42,4 +42,8 @@ rmcp = { version = "1.8", optional = true, default-features = false, features =
     "server",
     "transport-streamable-http-client-reqwest",
     "transport-streamable-http-server",
-] }
+] }
+
+[dev-dependencies]
+http-body-util = "0.1"
+tower = { version = "0.5", features = ["util"] }