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 ----
 
 

packages/core/ww_core/memory/assemble.py

@@ -104,6 +104,7 @@ def _build_volatile(
     foreshadows: list[ForeshadowView],
     digests: list[DigestView],
     beats: dict[str, Any],
+    directive: str | None = None,
 ) -> str:
     fore_lines = [
         f"【伏笔】{f.code} {f.title} [{f.status}]"
@@ -113,6 +114,8 @@ def _build_volatile(
         f"第{d.chapter_no}章：{_ser(d.facts)}" for d in sorted(digests, key=lambda d: d.chapter_no)
     ]
     sections = [
+        # 作者本章指令领衔——易变的每章输入，只入 volatile（断点后），守缓存前缀不变量 #9。
+        _section("本章指令", directive.strip()) if directive and directive.strip() else None,
         # 写章指令始终在场——保证 volatile（断点后的 input）永不为空（修空 prompt 400）。
         _section("写作指令", f"请创作第 {chapter_no} 章的正文。"),
         _section("本章注入卡片", cards),
@@ -130,11 +133,15 @@ async def assemble(
     recent_k: int = RECENT_DIGEST_COUNT,
     *,
     override: InjectionOverride | None = None,
+    directive: str | None = None,
 ) -> AssembledContext:
     """组装本章 prompt 上下文（确定性、可缓存）。
 
     `override`（B0 可控版作者覆盖）也是确定性输入：pin 强制纳入 / excluded 强制剔除 /
     recent_n 覆盖近况回看章数。draft 与注入读端点传同一覆盖，故「看到的=写章用的」（不变量 #6）。
+
+    `directive`（T4-b 本章指令）是临时的每章输入——只入 volatile（断点后），绝不污染
+    stable_core 缓存前缀（不变量 #9）；不持久化。
     """
     # recent_n 覆盖优先于默认 recent_k；非正值视为未设（守住至少回看若干章）。
     effective_k = override.recent_n if (override and override.recent_n) else recent_k
@@ -171,6 +178,8 @@ async def assemble(
 
     stable_core = _build_stable(spec, world_entities, main_characters, style, rules)
     cards = render_cards(selection, characters, world_entities)
-    volatile = _build_volatile(chapter_no, cards, foreshadows, recent_digests, outline.beats)
+    volatile = _build_volatile(
+        chapter_no, cards, foreshadows, recent_digests, outline.beats, directive
+    )
 
     return AssembledContext(stable_core=stable_core, volatile=volatile, selection=selection)