chore: release alkhttp 0.4.0

- version bump 0.3.0 -> 0.4.0 (the CONNECTION_CLOSED -> 503 gateway
  mapping is a wire-visible behavior change for HTTP callers matching
  on the previously-emitted 500 — a minor bump, not a patch)
- README usage snippet 0.3 -> 0.4
- changelog: cut [0.4.0] with the alkcall 0.7.0 wave entry
  (CF-005/006/007, no code change forced) + the CONNECTION_CLOSED
  mapping under Changed (wire-surface behavior)

Verification: cargo test (456 passed), cargo test --all-features
(589 passed), clippy -D warnings (default + all-features), fmt
--check, cargo publish --dry-run --allow-dirty

CHANGELOG.md

@@ -6,6 +6,8 @@ this crate adheres to [Semantic Versioning](https://semver.org/).
 
 ## [Unreleased]
 
+## [0.4.0] — 2026-09-07
+
 ### Changed
 
 - **`alkcall` dependency bumped to 0.7.0** (CF-005/006/007 — the
@@ -31,7 +33,9 @@ this crate adheres to [Semantic Versioning](https://semver.org/).
   (`from_wss`'s pending-call fail-all is retryable
   `CONNECTION_CLOSED`). The code is `retryable: true`, so the existing
   `retry_after` details path applies on 503. Tests pin the mapping and
-  the `Retry-After` passthrough.
+  the `Retry-After` passthrough. Behavioral break for HTTP callers
+  matching on the 500 previously emitted for this code — hence a minor
+  bump, not a patch.
 
 ## [0.3.0] — 2026-09-07
 

Cargo.lock

@@ -48,7 +48,7 @@ dependencies = [
 
 [[package]]
 name = "alkhttp"
-version = "0.3.0"
+version = "0.4.0"
 dependencies = [
  "alkcall",
  "arc-swap",

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "alkhttp"
-version = "0.3.0"
+version = "0.4.0"
 edition = "2021"
 rust-version = "1.85"
 license = "MIT OR Apache-2.0"

README.md

@@ -161,7 +161,7 @@ A lean single-side build takes `default-features = false` plus the side
 it needs:
 
 ```toml
-alkhttp = { version = "0.3", default-features = false, features = ["server"] }
+alkhttp = { version = "0.4", default-features = false, features = ["server"] }
 ```
 
 ## Security posture