feat(ux): T4-b 后端 — draft 端点接收本章指令 directive 直通 assemble(volatile)

assemble/_build_volatile 加 directive 参数（仅入 volatile，守缓存前缀不变量 #9）；stream_draft 加可选 body {directive} 透传；不持久化、无迁移；directive 长度入 log 不记原文。新 DraftStreamRequest schema + 重生成 TS 客户端。后端门禁绿: ruff/mypy/pytest。契约记 memory/contracts.md。

apps/api/ww_api/routers/projects.py

@@ -53,6 +53,7 @@ from ww_api.schemas.projects import (
     AcceptResponse,
     DraftResponse,
     DraftSaveRequest,
+    DraftStreamRequest,
     DraftView,
     ProjectCreateRequest,
     ProjectListResponse,
@@ -240,11 +241,17 @@ async def stream_draft(
     gateway: GatewayDep,
     injection_repo: InjectionRepoDep,
     session: Annotated[AsyncSession, Depends(get_session)],
+    body: DraftStreamRequest | None = None,
 ) -> StreamingResponse:
-    """流式写章草稿：组装记忆（含作者注入覆盖）→ 网关流 → 归一为 SSE 事件 → text/event-stream。"""
+    """流式写章草稿：组装记忆（含作者注入覆盖 + 本章指令）→ 网关流 → 归一 SSE → event-stream。
+
+    `body.directive`（可选，T4-b）是临时本章指令，直通 assemble→volatile（不持久化）；
+    无 body 的旧调用方仍可用（向后兼容）。
+    """
     request_id = getattr(request.state, "request_id", None)
+    directive = body.directive if body else None
     override = await injection_repo.get(project_id, chapter_no)
-    context = await assemble(repos, project_id, chapter_no, override=override)
+    context = await assemble(repos, project_id, chapter_no, override=override, directive=directive)
     log.info(
         "draft_stream_start",
         project_id=str(project_id),
@@ -252,6 +259,7 @@ async def stream_draft(
         request_id=request_id,
         stable_core_len=len(context.stable_core),
         volatile_len=len(context.volatile),
+        directive_len=len(body.directive) if body and body.directive else 0,
     )
 
     deltas = stream_chapter_draft(