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。

src/llm/slices/stats.py

@@ -0,0 +1,67 @@
+"""B - 攻防数据切片: 进球/射门/控球/xG 聚合(stats)。"""
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+from src.db.base import AsyncSessionLocal
+from src.llm.slices.common import MatchHeader, SliceResult, _is_stats_available
+from src.llm.slices.form import _get_form
+
+if TYPE_CHECKING:
+    from sqlalchemy.ext.asyncio import AsyncSession
+
+
+async def stats_slice(header: MatchHeader, *, limit: int = 10, before=None, db: AsyncSession | None = None) -> SliceResult:
+    """B - 攻防数据切片: 进球、射门、控球,评估攻防强度。before=match_date 用于回测。
+
+    db: 可选共享 session,避免每个切片独立建连(见 context_builder 模块 docstring)。
+    """
+    if db is not None:
+        home_form = await _get_form(db, header.home_team_id, before=before, limit=limit)
+        away_form = await _get_form(db, header.away_team_id, before=before, limit=limit)
+    else:
+        async with AsyncSessionLocal() as new_db:
+            home_form = await _get_form(new_db, header.home_team_id, before=before, limit=limit)
+            away_form = await _get_form(new_db, header.away_team_id, before=before, limit=limit)
+    lines = [f"── 攻防数据(近 {limit} 场) ──"]
+    n_total = 0
+    # P0-1 修复:每场历史比赛必须根据「该队当时是主是客」判断 side,
+    # 不能用本场 side 硬套 —— 否则进球/失球/xG 全部算反。
+    for label, name, form, team_id in (
+        ("主队", header.home_name, home_form, header.home_team_id),
+        ("客队", header.away_name, away_form, header.away_team_id),
+    ):
+        if form:
+            gf = ga = shots = sot = poss = xg = xga = 0
+            n = n_shots = n_poss = n_xg = 0
+            for fm in form:
+                if fm.home_goals is None: continue
+                is_home = (fm.home_team_id == team_id)
+                gf += fm.home_goals if is_home else fm.away_goals
+                ga += fm.away_goals if is_home else fm.home_goals
+                n += 1
+                # 只使用 cutoff 之前已可用的统计数据
+                if fm.stats and _is_stats_available(fm.stats, before):
+                    if fm.stats.home_shots is not None:
+                        shots += fm.stats.home_shots if is_home else fm.stats.away_shots
+                        sot += fm.stats.home_shots_on_target if is_home else fm.stats.away_shots_on_target
+                        n_shots += 1
+                    if fm.stats.home_possession is not None:
+                        poss += fm.stats.home_possession if is_home else (100 - fm.stats.home_possession)
+                        n_poss += 1
+                    if fm.stats.home_xg is not None:
+                        xg += fm.stats.home_xg if is_home else fm.stats.away_xg
+                        xga += fm.stats.away_xg if is_home else fm.stats.home_xg
+                        n_xg += 1
+            n_total += n
+            if n > 0:
+                lines.append(f"  {label} {name}:")
+                lines.append(f"    场均进球 {gf/n:.2f}, 场均失球 {ga/n:.2f}")
+                if n_shots: lines.append(f"    场均射门 {shots/n_shots:.1f}, 射正 {sot/n_shots:.1f}")
+                if n_poss: lines.append(f"    平均控球 {poss/n_poss:.1f}%")
+                if n_xg: lines.append(f"    场均 xG {xg/n_xg:.2f}, 场均被 xG {xga/n_xg:.2f}")
+            else:
+                lines.append(f"  {label} {name}: 无比分数据")
+        else:
+            lines.append(f"  {label} {name}: 无数据")
+    return SliceResult(text="\n".join(lines), has_data=n_total > 0, n_records=n_total)