Measure what each retrieval leg is worth before choosing a default

The product fuses a sparse page, a dense vector leg, and a picture leg that
only speaks when the words come back thin. Two of the three are off by
default, and until now no committed artifact said why.

bench/leg_check.py asks the frozen suite's 300 answerable queries as their
own holder, once per configuration, and prints recall, first-hit quality,
latency and per-modality recall in one table. The picture index and the
CLIP snapshot are 48 minutes of encoding, so they are carried in rather
than rebuilt per row; the harness refuses to run a leg whose snapshot is
missing instead of quietly scoring nothing.

The numbers that came back:
- words only, 10 atoms: recall@10 0.832, MRR 0.683, p50 64ms
- words only, the configured 24: 0.848, 0.688, 69ms
- words only, page 30: 0.855, 0.690, 67ms
- dense leg on: 0.845 / 0.858, MRR down, p50 469ms / 411ms
- picture leg available: 0.815 / 0.825, MRR 0.553 / 0.551, p50 148ms / 164ms

So both defaults stay off, and the acceptance bar (recall@10 >= 0.90) is
not met by the product path yet: 0.858 is the best measured row, the
in-repo counterpart at holder scope is 0.866. The loss concentrates in
picture golds, and the current CLIP leg pays more in dilution than it
returns there.

CHECKLIST p2-1 and p0-3.7 now quote the committed reports (DENSE.md,
QUANT.md, SCALE.md, TERMS.md) instead of scratch scripts.

bench/tests/test_leg_check.py

@@ -0,0 +1,56 @@
+"""The leg ablation must stay an ablation: every leg on and off, and a report that says so."""
+
+from __future__ import annotations
+
+from pathlib import Path
+from typing import Any
+
+from bench import leg_check
+
+
+def test_every_leg_appears_switched_both_ways() -> None:
+    dense = {config["dense"] for config in leg_check.CONFIGS}
+    visual = {config["visual"] for config in leg_check.CONFIGS}
+    assert dense == {True, False}, "the dense leg is only an ablation if both states are scored"
+    assert visual == {True, False}, "the picture leg is only an ablation if both states are scored"
+    pages = {int(config["top_k"]) for config in leg_check.CONFIGS}
+    assert leg_check.SHIPPED_TOP_K in pages, "the page size the product ships must be one of the rows"
+
+
+def test_the_report_prints_every_arm_with_its_numbers(tmp_path: Path) -> None:
+    arm = {
+        "label": "words only",
+        "top_k": 10,
+        "dense": False,
+        "visual": False,
+        "recall_at_10": 0.832,
+        "mrr": 0.683,
+        "hit_at_10": 0.833,
+        "wilson_lower": 0.787,
+        "p50_seconds": 0.0637,
+        "p95_seconds": 0.13,
+        "median_distinct_sources": 4,
+        "by_modality": {"code": 0.953, "pdf": 0.849},
+        "modality_counts": {"code": 128, "pdf": 86},
+    }
+    report: dict[str, Any] = {
+        "commit": "abcdef123456",
+        "schema_version": 8,
+        "store": "bench/runs/leg_check/data/ely_eye.sqlite",
+        "probes": 300,
+        "arms": [arm],
+        "machine": {
+            "system": "Linux",
+            "release": "6.18",
+            "cpus": 32,
+            "threads_per_query": 4,
+            "load_average_1_5_15": [1.0, 2.0, 3.0],
+        },
+    }
+    out = tmp_path / "LEGS.md"
+    leg_check.write_markdown(report, out)
+    text = out.read_text(encoding="utf-8")
+    assert "words only" in text
+    assert "0.832" in text and "63.7" in text, "recall and latency must sit in the same row"
+    assert "code" in text and "0.953" in text, "per-modality recall is where the picture legs show up"
+    assert "abcdef123456" in text and "load_average" not in text