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/standings.py

@@ -0,0 +1,81 @@
+"""D - 联赛排名切片: 积分榜位置与实力差距(standings)。"""
+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.llm.slices.common import MatchHeader, SliceResult
+
+if TYPE_CHECKING:
+    from sqlalchemy.ext.asyncio import AsyncSession
+
+
+async def standings_slice(header: MatchHeader, *, before=None, db: AsyncSession | None = None) -> SliceResult:
+    """D - 联赛排名切片: 两队积分榜位置、积分、近期走势(form)、分区,评估整体实力差距。
+
+    before 参数保留与其他切片一致的签名(积分榜是最新快照,无历史版本,不受 cutoff 影响)。
+    db: 可选共享 session(见 context_builder 模块 docstring)。
+
+    语义区分:
+      - 两队都有积分榜行 → has_data=True(明确的排名信息)
+      - 任一队缺失 → has_data=False(升班马/杯赛无榜,信息不完整时明确声明)
+    """
+    from src.db.models import League, Standing
+
+    if db is not None:
+        league = (await db.execute(select(League).where(League.id == header.league_id))).scalar_one_or_none()
+        rows = (
+            (
+                await db.execute(
+                    select(Standing)
+                    .options(selectinload(Standing.team))
+                    .where(Standing.league_id == header.league_id)
+                    .order_by(Standing.position.asc())
+                )
+            )
+            .scalars()
+            .all()
+            if league
+            else []
+        )
+    else:
+        async with AsyncSessionLocal() as new_db:
+            return await standings_slice(header, before=before, db=new_db)
+
+    lines = [f"── 联赛排名({header.league_name} 共 {len(rows)} 队) ──"]
+    n_records = 0
+
+    def _fmt(row) -> str:
+        zg = f" xG差 {row.xgd:+.1f}" if row.xg_for is not None and row.xg_against is not None and row.goal_diff is not None else ""
+        form = f" 近5场 {row.form}" if row.form else ""
+        zone = f" [{row.zone}]" if row.zone else ""
+        return (
+            f"    第 {row.position} 名: {row.points} 分 / {row.played} 场 "
+            f"({row.won}胜{row.drawn}平{row.lost}负, 进{row.goals_for}失{row.goals_against} 净胜{row.goal_diff:+d}"
+            f"{zg}){form}{zone}"
+        )
+
+    for label, team_id in (("主队", header.home_team_id), ("客队", header.away_team_id)):
+        row = next((r for r in rows if r.team_id == team_id), None)
+        if row is None:
+            lines.append(f"  {label}: 暂无积分榜数据(可能杯赛/赛季未开始)")
+        else:
+            n_records += 1
+            lines.append(f"  {label} {header.home_name if label == '主队' else header.away_name}:")
+            lines.append(_fmt(row))
+
+    # 两队排名对比摘要
+    home_row = next((r for r in rows if r.team_id == header.home_team_id), None)
+    away_row = next((r for r in rows if r.team_id == header.away_team_id), None)
+    if home_row and away_row:
+        diff = home_row.position - away_row.position  # 正数=主队排名更靠前(名次更小)
+        lead = f"主队排名高 {diff} 位" if diff > 0 else (f"客队排名高 {-diff} 位" if diff < 0 else "两队同排名结构")
+        pts_diff = home_row.points - away_row.points
+        lines.append(f"  排名对比: {lead}, 分差 {pts_diff:+d}")
+
+    # has_data: 两队都有行才算完整;只有一队时仍有价值,但标记不完整
+    has_data = n_records >= 1
+    return SliceResult(text="\n".join(lines), has_data=has_data, n_records=n_records)