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.

Cargo.toml

@@ -14,39 +14,58 @@ exclude = [".opencode/", "AGENTS.md", "docs/", "tasks/", "Cargo.lock"]
 name = "alkhttp"
 
 [features]
-default = ["h2", "http1"]
+default = ["server", "client", "h2", "http1"]
+# Producer side: the axum `Router` host (gateway routes, /healthz,
+# /openapi.json, /mcp, the WS upgrade path) and the hyper connection
+# driver. `openapi` rides along because /openapi.json is part of the
+# default surface (ADR-042/045); `http1` rides along so the hyper
+# auto-builder always has at least one protocol compiled.
+server = ["dep:axum", "dep:hyper", "dep:hyper-util", "dep:uuid", "openapi", "http1"]
+# Consumer side: the shared outbound client host and the HTTP-backed
+# import adapters (`from_jsonschema`, `from_openapi` with `openapi`,
+# `from_mcp` with `mcp`).
+client = ["dep:arc-swap", "dep:httpdate", "dep:percent-encoding", "dep:reqwest", "dep:reqwest-middleware", "dep:reqwest-retry", "dep:url"]
+# Shared OpenAPI document model: `from_openapi` parsing (client side)
+# and the `to_openapi` gateway projection (server side).
+openapi = ["dep:yaml_serde"]
+# MCP adapters: `from_mcp` needs `client`, `to_mcp` needs `server`.
 mcp = ["dep:rmcp"]
+# WebSocket transport (from_wss consumer; tungstenite flavor of the
+# WS ↔ byte-stream adapter).
 wss = ["dep:tokio-tungstenite"]
-test-support = ["dep:tokio-tungstenite"]
-h2 = ["dep:hyper", "hyper-util/http2", "hyper/http2"]
-http1 = ["dep:hyper", "hyper-util/http1", "hyper/http1"]
+# Test-only WS client helpers (downstream deployment tests); implies
+# `server` — the helpers live with the upgrade path.
+test-support = ["server", "dep:tokio-tungstenite"]
+# HTTP protocol features of the server connection driver (ADR-001).
+h2 = ["server", "dep:hyper", "hyper-util/http2", "hyper/http2"]
+http1 = ["server", "dep:hyper", "hyper-util/http1", "hyper/http1"]
 
 [dependencies]
 alkcall = { version = "0.2", features = ["gateway"] }
-arc-swap = "1"
-axum = { version = "0.8", features = ["ws"] }
+arc-swap = { version = "1", optional = true }
+axum = { version = "0.8", optional = true, features = ["ws"] }
+bytes = "1"
+futures = "0.3"
+http = "1"
+httpdate = { version = "1", optional = true }
 hyper = { version = "1", optional = true, features = ["server"] }
-hyper-util = { version = "0.1", features = ["server", "service", "tokio"] }
-httpdate = "1"
+hyper-util = { version = "0.1", optional = true, features = ["server", "service", "tokio"] }
 jsonschema = { version = "0.46", default-features = false }
-reqwest = { version = "0.13", default-features = false, features = ["json", "stream", "rustls"] }
-reqwest-middleware = "0.5"
-reqwest-retry = "0.9"
+parking_lot = "0.12"
+percent-encoding = { version = "2", optional = true }
+reqwest = { version = "0.13", optional = true, default-features = false, features = ["json", "stream", "rustls"] }
+reqwest-middleware = { version = "0.5", optional = true }
+reqwest-retry = { version = "0.9", optional = true }
 tokio = { version = "1", features = ["macros", "rt-multi-thread", "io-util", "net", "fs", "time", "sync"] }
 tokio-tungstenite = { version = "0.29", optional = true, default-features = false, features = ["connect", "rustls-tls-webpki-roots", "handshake"] }
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
-yaml_serde = "0.10"
 async-trait = "0.1"
 tracing = "0.1"
 thiserror = "2"
-uuid = { version = "1", features = ["v4"] }
-futures = "0.3"
-http = "1"
-http-body-util = "0.1"
-url = "2"
-percent-encoding = "2"
-parking_lot = "0.12"
+uuid = { version = "1", optional = true, features = ["v4"] }
+url = { version = "2", optional = true }
+yaml_serde = { version = "0.10", optional = true }
 rmcp = { version = "1.8", optional = true, default-features = false, features = [
     "client",
     "server",
@@ -75,8 +94,24 @@ required-features = ["test-support"]
 
 [[test]]
 name = "from_mcp_integration"
-required-features = ["mcp"]
+required-features = ["server", "client", "mcp"]
 
 [[test]]
 name = "full_surface"
-required-features = ["mcp", "test-support"]
+required-features = ["server", "client", "openapi", "mcp", "test-support"]
+
+[[test]]
+name = "client_config_reload"
+required-features = ["client"]
+
+[[test]]
+name = "client_tls"
+required-features = ["client"]
+
+[[test]]
+name = "retry_after_budget"
+required-features = ["client"]
+
+[[test]]
+name = "retry_policy_wire"
+required-features = ["client"]