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/form.py

@@ -0,0 +1,85 @@
+"""A - 近期状态切片: 近 N 场赛果 / 走势(form)。"""
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+from sqlalchemy import select
+from sqlalchemy.orm import selectinload
+
+from src.db.base import AsyncSessionLocal
+from src.db.models import Match
+from src.llm.slices.common import MatchHeader, SliceResult, _is_stats_available, _outcome
+
+if TYPE_CHECKING:
+    from sqlalchemy.ext.asyncio import AsyncSession
+
+
+async def form_slice(header: MatchHeader, *, limit: int = 5, before=None, db: AsyncSession | None = None) -> SliceResult:
+    """A - 近期状态切片: 两队近 N 场赛果、关键事件、走势判断。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 = []
+    n_scored = 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 —— 否则客场输球会被算成主场赢球。
+    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),
+    ):
+        lines.append(f"── {label}近况({name},近 {limit} 场) ──")
+        if form:
+            wins = draws = losses = 0
+            for fm in form:
+                is_home = (fm.home_team_id == team_id)
+                side = "home" if is_home else "away"
+                o = _outcome(fm.home_goals, fm.away_goals, side)
+                if o == "W": wins += 1
+                elif o == "D": draws += 1
+                else: losses += 1
+                if fm.home_goals is not None:
+                    n_scored += 1
+                score = f"{fm.home_goals}-{fm.away_goals}" if fm.home_goals is not None else "vs"
+                xg = ""
+                if fm.stats and _is_stats_available(fm.stats, before) and fm.stats.home_xg is not None:
+                    own = fm.stats.home_xg if is_home else fm.stats.away_xg
+                    xg = f" (xG {own:.1f})"
+                opp = fm.away_team.name if is_home else fm.home_team.name
+                lines.append(f"  {o} {score} vs {opp}{xg}")
+            lines.append(f"  近 {len(form)} 场: {wins}胜 {draws}平 {losses}负")
+        else:
+            lines.append("  无数据")
+    return SliceResult(text="\n".join(lines), has_data=n_scored > 0, n_records=n_scored)
+
+
+async def _get_form(db, team_id: int, before, *, limit: int = 5) -> list[Match]:
+    """某队近 N 场(已完赛)。before=None 表示不限制(预测赛前的场景由调用方保证)。
+
+    必须预加载 stats / home_team / away_team:切片函数会读取这些关系,
+    而 async session 下惰性加载会抛 MissingGreenlet。
+    (models.py 已声明 lazy="selectin",此处显式声明以固化查询意图。)
+    """
+    stmt = (
+        select(Match)
+        .options(
+            selectinload(Match.stats),
+            selectinload(Match.home_team),
+            selectinload(Match.away_team),
+        )
+        .where(Match.match_status == "finished")
+        .where(Match.home_goals.is_not(None))
+        .where((Match.home_team_id == team_id) | (Match.away_team_id == team_id))
+        .order_by(Match.match_date.desc())
+        .limit(limit)
+    )
+    if before is not None:
+        stmt = stmt.where(Match.match_date < before)
+    result = await db.execute(stmt)
+    return list(result.scalars().all())