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_p0_home_away.py

@@ -101,7 +101,7 @@ class TestFormSliceHomeAwayIdentity:
             home_name="曼城",
             away_name="利物浦",
         )
-        import src.llm.context_builder as cb
+        import src.llm.slices.form as cb
         orig_get_form = cb._get_form
         async def mock_get_form(db, team_id, before, *, limit):
             return [hist_match] if team_id == 1 else []
@@ -132,7 +132,7 @@ class TestFormSliceHomeAwayIdentity:
             home_name="阿森纳",
             away_name="切尔西",
         )
-        import src.llm.context_builder as cb
+        import src.llm.slices.form as cb
         orig_get_form = cb._get_form
         async def mock_get_form(db, team_id, before, *, limit):
             return [hist_match] if team_id == 2 else []
@@ -169,7 +169,7 @@ class TestStatsSliceHomeAwayIdentity:
             stats=_make_stats(home_xg=2.5, away_xg=0.8, home_shots=15, away_shots=5,
                               home_sot=6, away_sot=2, home_poss=60.0),
         )
-        import src.llm.context_builder as cb
+        import src.llm.slices.stats as cb
         orig_get_form = cb._get_form
         async def mock_get_form(db, team_id, before, *, limit):
             return [hist_match] if team_id == 1 else []