feat(build): feature-sided builds — server/client sides independently selectable (ADR-039 Amendment 1)

Split the feature graph so consumers pulling only the import adapters
(from_openapi / from_jsonschema / from_mcp) no longer compile the axum
/ hyper server stack, and server-only deployments no longer compile
reqwest. One crate, one import path — sides cut by features, not by a
crate split.

Feature graph:
- server (default): axum host, gateway, WS upgrade, to_openapi, to_mcp
- client (default): client host, forward, from_jsonschema, from_openapi
- openapi: shared OpenAPISpec model (implied by both sides)
- mcp: from_mcp needs client, to_mcp needs server
- wss: tungstenite transport (from_wss); tungstenite half of the
  shared WS↔byte-stream adapter
- h2/http1: hyper protocol features; imply server

Wire-contract neutral: gateway endpoints, ALPNs, and all public API
shapes unchanged; defaults keep both sides on.

Supporting changes:
- forward.rs drops its axum::body::Bytes type leak (bytes crate types)
- bounded_join + error-echo caps move to input_validation (usable by
  both sides; openapi_spec no longer imports from forward)
- byte_adapter: axum flavor compiles under server, tungstenite under
  wss; the generic pumps stay shared (WS-11)
- input_validation / openapi_spec import-only internals gated to the
  side that consumes them
- http-body-util moves to dev-dependencies (was test-only)
- integration-test required-features updated for the new sides
- from_wss unit tests (axum producer harness) gated to server

Verified: cargo test (defaults, 453) and --all-features (575) pass;
lean side builds (client / server / client,mcp / client,wss /
server,wss / openapi-only) build clean with zero warnings;
clippy -D warnings clean across all feature combinations; fmt clean.

docs/architecture/decisions/070-from-wss-consumer-adapter.md

@@ -40,7 +40,8 @@ same-protocol importer, with WSS as the transport instead of QUIC.**
 
 - **Feature gate:** `wss = ["dep:tokio-tungstenite"]` (not default —
   a process that never consumes over WSS should not compile a WS
-  client).
+  client). `from_wss` itself additionally requires the `client`
+  feature side (ADR-039 Amendment 1).
 - **Shape:** implements `OperationAdapter`
   ([ADR-017](017-call-protocol-client-and-adapter-contract.md); record:
   alkcall ADR-022):