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。

packages/core/tests/test_memory.py

@@ -293,6 +293,18 @@ async def test_assemble_override_recent_n_limits_digests() -> None:
     assert "实体1" not in names
 
 
+async def test_assemble_directive_in_volatile_not_stable() -> None:
+    repos = build_repos(
+        characters=[char("林动", role="主角")],
+        spec=ProjectSpecView(title="书"),
+    )
+    ctx = await assemble(repos, PROJECT, 1, directive="多写战斗")
+    assert "本章指令" in ctx.volatile
+    assert "多写战斗" in ctx.volatile
+    assert "多写战斗" not in ctx.stable_core
+    assert "本章指令" not in ctx.stable_core
+
+
 # ---- render_cards ----