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>

docs/12-13-serving.md

@@ -0,0 +1,98 @@
+# Phase 12+13: Continuous Batching + HTTP API — Design Document (Milestone ③)
+
+## Goal
+
+实现 HTTP serving 层：接收请求、调度执行、streaming 返回结果。OpenAI 兼容 API。
+
+由于当前是单请求引擎（无 multi-GPU、无并发），Phase 12 (continuous batching) 和 Phase 13 (HTTP API) 合并实现：先实现单请求 serving，scheduler 作为 placeholder 留待后续扩展。
+
+## Architecture
+
+```
+Client (curl / OpenAI SDK)
+    │
+    ▼  HTTP POST /v1/chat/completions
+┌─────────────────────────────────────┐
+│  xserv-api (axum)                   │
+│  - Parse request                    │
+│  - Apply chat template              │
+│  - Submit to engine via channel     │
+│  - Stream SSE chunks from channel   │
+└────────────┬────────────────────────┘
+             │  InferenceRequest → mpsc channel
+             ▼
+┌─────────────────────────────────────┐
+│  xserv-engine (dedicated thread)    │
+│  - Receive requests                 │
+│  - Run model forward (prefill+decode)│
+│  - Send tokens back via channel     │
+└─────────────────────────────────────┘
+```
+
+## Crates
+
+- `xserv-engine`: inference orchestration (model + cache + generate loop)
+- `xserv-api`: HTTP server with axum
+
+Both in one binary: `xserv-server`
+
+## API Endpoints
+
+```
+POST /v1/chat/completions   # main endpoint
+GET  /v1/models             # list models
+GET  /health                # health check
+```
+
+## Request/Response (OpenAI compatible)
+
+Request:
+```json
+{
+  "model": "qwen3-8b",
+  "messages": [{"role": "user", "content": "Hello"}],
+  "stream": true,
+  "max_tokens": 256,
+  "temperature": 1.0
+}
+```
+
+SSE Response:
+```
+data: {"id":"...","choices":[{"delta":{"content":"Hi"},"index":0}]}
+
+data: {"id":"...","choices":[{"delta":{},"finish_reason":"stop"}]}
+
+data: [DONE]
+```
+
+## Engine Design
+
+```rust
+pub struct Engine {
+    model: Qwen3,
+    config: ModelConfig,
+    tokenizer: Tokenizer,
+}
+
+impl Engine {
+    pub fn generate(&self, prompt_tokens: &[u32], params: &SamplingParams,
+                    sender: mpsc::Sender<Token>) { ... }
+}
+```
+
+Engine runs on a dedicated OS thread (avoids async/GPU conflicts).
+API handlers communicate via `tokio::sync::mpsc` channels.
+
+## Sampling
+
+For this phase: greedy only (temperature=0 or 1 with argmax).
+Top-k/top-p sampling added later.
+
+## Test Plan
+
+- [ ] curl streaming request → get SSE chunks
+- [ ] Python OpenAI SDK client works
+- [ ] /v1/models returns model info
+- [ ] /health returns 200
+- [ ] Multiple sequential requests work