docs: missing_docs sweep — 0 warnings + deny gate + publish-prep decisions (HY-02, HY-04, HY-11)

- document every public-API item across 18 files (openapi_spec model,
  HttpAuthScheme/HttpServiceConfig, HttpClientBuildError + SharedHttpClient
  accessors, RetryAfterMiddleware, GatewayDispatch, gateway error
  mapping, CallRequest/SchemaQuery/SubscribeStream, HttpAdapter +
  ALPNs + builders, decoy/healthz/state, WsSessions/WsPumps,
  from_openapi/from_jsonschema/from_mcp/from_wss/to_mcp, lib.rs module
  docs)
- enforcement: #![deny(missing_docs)] at crate root — stronger than CI
  rustdocflags (every build incl. cfg(test), where rustdoc misses the
  test-support module docs)
- HY-10 (opportunistic): all 8 docs.rs/alkhttp placeholder ADR links +
  the one relative ../docs link converted to plain text; the 10
  pre-existing private/redundant intra-doc-link warnings fixed —
  RUSTDOCFLAGS="-D warnings" cargo doc is fully clean
- HY-11 decision: docs/ + tasks/ excluded from the published package
  (contributor-facing design/process material; ADR references degrade
  to plain text uniformly). cargo publish --dry-run: 38 files, ~889 KiB,
  zero docs/ or tasks/ entries
- HY-04 decision: keep + document — frame_channel0_chunk's unwrap is
  on serializing the acyclic EventEnvelope (unreachable failure);
  # Panics on it and the adjacent WsClient senders state the contract

Verified: cargo test (299 + 5 TLS), --all-features (370 + suites),
--no-default-features (299), clippy --all-targets -D warnings
(default + all-features), fmt --check, cargo doc -D warnings clean,
cargo publish --dry-run --allow-dirty clean.

Tasks: review-001-missing-docs-sweep (final pending task; 42/42)

src/adapters/to_mcp.rs

@@ -119,15 +119,20 @@ fn batch_input_schema() -> Value {
     })
 }
 
+/// The `to_mcp` projection (ADR-041): the local call-protocol
+/// operations exposed as 4 fixed MCP gateway tools (`search`, `schema`,
+/// `call`, `batch`) over the streamable-HTTP transport.
 pub struct ToMcpGateway {
     dispatch: Arc<GatewayDispatch>,
 }
 
 impl ToMcpGateway {
+    /// Assemble the gateway over a dispatch spine.
     pub fn new(dispatch: Arc<GatewayDispatch>) -> Self {
         Self { dispatch }
     }
 
+    /// The dispatch spine the tools invoke through.
     pub fn dispatch(&self) -> &Arc<GatewayDispatch> {
         &self.dispatch
     }
@@ -480,8 +485,11 @@ impl rmcp::handler::server::ServerHandler for ToMcpGateway {
     }
 }
 
+/// The ready-to-mount rmcp service wrapping [`ToMcpGateway`] over the
+/// streamable-HTTP transport.
 pub type ToMcpService = StreamableHttpService<ToMcpGateway, LocalSessionManager>;
 
+/// Build the mountable MCP service for a dispatch spine.
 pub fn to_mcp_service(dispatch: Arc<GatewayDispatch>) -> ToMcpService {
     let gateway = ToMcpGateway::new(dispatch);
     StreamableHttpService::new(