refactor: context_builder 按 slice 拆到 src/llm/slices/ 包

单文件拆分(仅搬迁无逻辑修改):
- common.py    共享类型/头信息/_outcome/_is_stats_available
- form.py      form_slice + _get_form
- h2h.py       h2h_slice + _get_h2h
- stats.py     stats_slice(复用 form._get_form)
- home_away.py home_away_slice + _get_home_away
- standings.py standings_slice
- aggregate.py build_context

context_builder.py 改为纯 re-export 门面,公开签名不变。
同步修复测试 patch 目标(p0_home_away/h2h_perspective/multi_agent_cutoff)
与 regressions 源码断言(读 slices/*.py)。

全量测试 270 通过。

tests/test_h2h_perspective.py

@@ -70,7 +70,7 @@ class TestH2HCurrentHomePerspective:
             _make_h2h_match(101, home_id=2, away_id=1, home_goals=3, away_goals=1,
                             home_name="阿森纳", away_name="利物浦"),
         ]
-        import src.llm.context_builder as cb
+        import src.llm.slices.h2h as cb
         orig = cb._get_h2h
         async def mock_get_h2h(db, home_id, away_id, before, *, limit):
             return matches
@@ -100,7 +100,7 @@ class TestH2HCurrentHomePerspective:
             _make_h2h_match(201, home_id=1, away_id=2, home_goals=2, away_goals=1,
                             home_name="曼城", away_name="诺维奇"),
         ]
-        import src.llm.context_builder as cb
+        import src.llm.slices.h2h as cb
 
         async def mock_get_h2h(db, h, a, before, **kw):
             # 真实契约是 async(见 context_builder.py 的 `h2h = await _get_h2h(...)`),
@@ -122,7 +122,7 @@ class TestH2HCurrentHomePerspective:
             _make_h2h_match(301, home_id=2, away_id=1, home_goals=0, away_goals=2,
                             home_name="热刺", away_name="切尔西"),  # 切尔西客场 2-0 赢
         ]
-        import src.llm.context_builder as cb
+        import src.llm.slices.h2h as cb
         orig = cb._get_h2h
         async def mock_get_h2h(db, h, a, before, *, limit):
             return matches