feat(nevo): backfill leaked preamble + ratio gold-set benchmark (#86)

#86.2 backfill + #86.3 benchmark, plus a #86.1 over-strip fix found en route.

extractor.py
- extract_nevo_ratio(): capture Nevo's מיני-רציו block (editorial holdings
  summary) before it is stripped — a free professional gold-set (#86.3).
- _DECISION_START hardening (#86.2): the merged #86.1 regex over-stripped.
  (a) פסק-דין headers are markdown-wrapped (**פסק  דין**); the old anchor
      required the keyword as the first line char with one separator, so it
      missed the header and matched a citation 32K deep (עמ"נ 50567-07-21,
      losing 45% of the body). Now tolerates leading markdown + 0-3 seps,
      and the final-nun form (דין ן vs דינו נ).
  (b) bare השופט/הנשיא matched CITATIONS ("השופט מ' חשין, פסקה 23"). The
      authoring-judge line ends with a colon; we now require it.

ingest.py
- capture the ratio before stripping and store it on the row (best-effort,
  non-fatal); also strip the text-upload path (was file-only).

db.py
- add case_law.nevo_ratio column (additive); allow it in update_case_law.

scripts/backfill_nevo_preamble.py (#86.2) — dry-run-by-default data migration:
finds historically-leaked rulings, captures ratio→nevo_ratio, rewrites
full_text (+content_hash), reindexes, and FLAGS (never deletes) halachot whose
quote lives in the removed preamble (review_status=pending_review +
nevo_preamble_leak flag). Safety guard: rows with keep%<--min-keep (60) are
excluded from --apply as suspected over-strip. --apply writes backup+manifest
to data/audit/ first. Chair-gated — NOT applied here.

scripts/nevo_ratio_benchmark.py (#86.3) — LLM-as-judge (local claude_session,
zero cost) measures recall/precision/granularity of our halachot vs the Nevo
ratio. Works pre- and post-backfill (reads nevo_ratio, falls back to full_text).

Verified:
- pytest tests/test_nevo_preamble.py — 12 passed (incl. citation/markdown
  over-strip regressions).
- backfill dry-run: 19 leaked rulings, 27 contaminated halachot, all ≥75%
  keep (the 32K over-strip is gone).
- benchmark on בג"ץ 1764/05: recall=0.875 precision=1.0 granularity=1.75x.

Invariants: G1 (normalize at source — strip/capture at ingest, not at read);
no silent swallow (contaminated halachot flagged + reported, not dropped);
data-migration is dry-run-default with backup+manifest, chair-gated.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

mcp-server/tests/test_nevo_preamble.py

@@ -55,3 +55,64 @@ def test_markers_past_400_chars_still_detected():
     text = header + _PREAMBLE + "השופטת ע' ארבל:\n\nגוף ההחלטה..."
     out = ex.strip_nevo_preamble(text)
     assert out.startswith("השופטת ע' ארבל:")
+
+
+# ── extract_nevo_ratio (#86.3 gold-set capture) ──
+
+def test_extract_ratio_returns_block_before_body():
+    text = _PREAMBLE + "השופט ס' ג'ובראן:\n\nגוף ההחלטה..."
+    ratio = ex.extract_nevo_ratio(text)
+    assert "העותרים לא הוכיחו טעם מיוחד" in ratio
+    assert "המחוקק הגביל את הזמן" in ratio
+    # must not bleed into the judgment body
+    assert "גוף ההחלטה" not in ratio
+    assert "השופט ס' ג'ובראן" not in ratio
+
+
+def test_extract_ratio_stops_at_following_marker():
+    # ratio first, then a bibliography marker AFTER it
+    text = (
+        "מיני-רציו:\n* עיקרון אחד בלבד.\n\n"
+        "פסקי דין שאוזכרו:\nבג\"ץ 1/00\n\n"
+        "פסק-דין\nגוף..."
+    )
+    ratio = ex.extract_nevo_ratio(text)
+    assert "עיקרון אחד בלבד" in ratio
+    assert "פסקי דין שאוזכרו" not in ratio
+    assert "בג\"ץ 1/00" not in ratio
+
+
+def test_extract_ratio_empty_when_no_marker():
+    assert ex.extract_nevo_ratio("פסק דין\nהשופט כהן: ...") == ""
+    assert ex.extract_nevo_ratio("") == ""
+
+
+# ── #86.2 over-strip regressions ──
+
+def test_citation_judge_line_is_not_a_decision_start():
+    # "השופט מ' חשין, פסקה 23" is a CITATION (comma, no colon) — must NOT be
+    # treated as the decision opening, or 32K of real body gets stripped.
+    body = (
+        "**פסק דין**\n\n"
+        "שני ערעורים לפניי. כפי שנפסק מפי כבוד \n\n"
+        "השופט מ' חשין, פסקה 23 (להלן עניין קהתי), יש לבחון...\n"
+    )
+    text = _PREAMBLE + body
+    out = ex.strip_nevo_preamble(text)
+    assert out.startswith("**פסק דין**")
+    assert "השופט מ' חשין, פסקה" in out  # citation kept inside body
+    assert "מיני-רציו" not in out
+
+
+def test_markdown_wrapped_pdin_header_is_stripped():
+    text = _PREAMBLE + "**פסק  דין**\n\nשני ערעוריה הנדונים..."
+    out = ex.strip_nevo_preamble(text)
+    assert out.startswith("**פסק  דין**")
+    assert "מיני-רציו" not in out
+
+
+def test_author_line_with_colon_still_strips():
+    text = _PREAMBLE + "כב' השופטת ד' ברק-ארז:\n\nגוף ההחלטה..."
+    out = ex.strip_nevo_preamble(text)
+    assert out.startswith("כב' השופטת ד' ברק-ארז:")
+    assert "מיני-רציו" not in out