feat(web): 角色群像按定位配比生成（role_mix 缩减版）

- 契约: CharacterGenerateRequest 加 role_mix{定位:数量}，model_validator 归一
  count=各值之和（每项≥1、总和≤12），缺省退回单一 role+count
- @llm: build_character_gen_context/run_character_gen 加 role_mix 参并注入配比块
  （确定性、无时间戳，守 assemble 纯函数）；router 穿参
- character-gen.md 加"按配比严格分配 role"纪律 + regen prompt_hashes 金标准
- 前端: sanitizeRoleMix/roleMixTotal + buildCharacterGenerateRequest 第4参 +
  useCharacterGen 穿参 + RoleMixEditor + CharacterGenerator 单一/配比双模

apps/web/lib/generation/useCharacterGen.test.ts

@@ -53,6 +53,24 @@ describe("useCharacterGen", () => {
     expect(toast).not.toHaveBeenCalled();
   });
 
+  it("按定位配比生成：请求体带 role_mix、count 取各值之和", async () => {
+    post.mockResolvedValue({ data: { cards: [] }, error: null });
+    const { result } = renderHook(() => useCharacterGen());
+    await act(async () => {
+      await result.current.generate({
+        projectId: "p1",
+        brief: "群像",
+        count: 1,
+        role: "忽略",
+        roleMix: { 主角: 1, 对手: 2 },
+      });
+    });
+    const body = post.mock.calls[0]?.[1]?.body;
+    expect(body.role_mix).toEqual({ 主角: 1, 对手: 2 });
+    expect(body.count).toBe(3);
+    expect(body.role).toBeUndefined();
+  });
+
   it("生成成功但 cards 缺失：回退为空数组", async () => {
     post.mockResolvedValue({ data: { cards: null }, error: null });
     const { result } = renderHook(() => useCharacterGen());