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/client/retry_after.rs

@@ -61,6 +61,10 @@ fn parse_retry_after_with_ceiling(value: &str, ceiling: Duration) -> Option<Syst
     Some(clamped)
 }
 
+/// Per-URL `Retry-After` memory (FWD-05): records the backlog deadline
+/// an upstream declared and holds back subsequent requests to that URL
+/// until it elapses. LRU-bounded by capacity; deadlines clamped to the
+/// configured ceiling.
 pub struct RetryAfterMiddleware {
     deadlines: Mutex<HashMap<Url, SystemTime>>,
     capacity: usize,
@@ -68,10 +72,22 @@ pub struct RetryAfterMiddleware {
 }
 
 impl RetryAfterMiddleware {
+    /// A middleware with the default 300 s `Retry-After` ceiling.
+    ///
+    /// # Panics
+    ///
+    /// Panics when `capacity` is 0 (a rate-limit memory that remembers
+    /// nothing is a construction error, not a runtime fallback).
     pub fn with_capacity(capacity: usize) -> Self {
         Self::with_capacity_and_ceiling(capacity, Duration::from_secs(300))
     }
 
+    /// A middleware with an explicit `capacity` bound and a ceiling
+    /// clamping hostile upstream deadlines.
+    ///
+    /// # Panics
+    ///
+    /// Panics when `capacity` is 0.
     pub fn with_capacity_and_ceiling(capacity: usize, ceiling: Duration) -> Self {
         Self {
             deadlines: Mutex::new(HashMap::with_capacity(capacity.min(128))),