phase 12+13: HTTP API server with OpenAI-compatible endpoint (Milestone ③)

New crate: xserv-server
- Engine thread: loads Qwen3-8B, processes requests sequentially
- axum HTTP server: /health, /v1/models, /v1/chat/completions
- tokio::sync::mpsc channel between API and engine threads
- Non-streaming JSON response (streaming SSE to be added later)

API is OpenAI-compatible:
  POST /v1/chat/completions {"messages": [...], "max_tokens": N}
  → {"choices": [{"message": {"content": "..."}}]}

Verified: "Hi" → ", I'm" (3 tokens), model runs correctly via HTTP.

Key learnings:
- std::sync::mpsc::SyncSender is Send but NOT Sync → wrap in Mutex for Arc<AppState>
- MutexGuard must not live across await points (scope carefully)
- axum 0.8 Extension<Arc<T>> requires T: Send + Sync

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/xserv-server/Cargo.toml

@@ -0,0 +1,21 @@
+[package]
+name = "xserv-server"
+version.workspace = true
+edition.workspace = true
+
+[[bin]]
+name = "xserv-server"
+path = "src/main.rs"
+
+[dependencies]
+xserv-cuda = { path = "../xserv-cuda" }
+xserv-tensor = { path = "../xserv-tensor" }
+xserv-kernels = { path = "../xserv-kernels" }
+xserv-model = { path = "../xserv-model" }
+xserv-tokenizer = { path = "../xserv-tokenizer" }
+half.workspace = true
+serde.workspace = true
+serde_json.workspace = true
+tokio.workspace = true
+axum.workspace = true
+uuid.workspace = true