feat(lv-mccarthy): pre-register the v2 gate before any arm is generated

Freezes the lv-mccarthy gate design while no McCarthy generation exists, per the
lv-hemingway precedent: a threshold chosen after seeing the numbers is not a
threshold. Three arms (base-unadapted, ckpt900, ckpt450), 60 beats, 4 seeds.

Settles the voice-axis question D1 deferred. McCarthy's corpus measures 0.0 quote
marks per 10k against Hemingway's 838, so "emit no quotation marks" is the cheapest
way to move a char-bigram Delta without learning a sentence. Three parts:

  PRIMARY    unchanged -- the mccarthy register names the punctuation and
             --system-from drives the base control with the same prompt, so the
             cheap win is handed to both sides.
  SECONDARY  voice_distance.py --secondary-normalised re-runs the whole analysis
             with punctuation stripped from the reference and every arm. A
             conservative lower bound; reported, never the verdict.
  TRIGGER    voice_distance.py --punct-report evaluates a pre-registered line --
             base-unadapted quote density above 100 per 10k means the control did
             not take the win it was handed, and the normalised read is promoted
             to load-bearing. 100 is the order-of-magnitude line between this
             corpus's 0.0 and Hemingway's 838, chosen now rather than after the
             table prints.

ckpt450 is NOT tied with the minimum (+4.9x the 0.00393 median neighbour jitter)
and the pre-registration says so. It is generated to test a stated prior and to
price the memorisation headroom an earlier checkpoint buys on an in-copyright
author, with a decision rule that says exactly what result would let it displace
ckpt900.

Instrument controls, run before this landed:
  - the voice_distance.py refactor reproduces the shipped lv-hemingway
    voice_distance.txt BYTE FOR BYTE on the default path
  - strip_punct drops a quote-bearing sample from 2500.0 to 0.0 marks per 10k
  - the secondary read still resolves a gap on the Hemingway arms (+0.410 at
    7.3x floor), so a null on McCarthy would be a finding, not a blind detector

Pre-flight re-run at gate time rather than quoted from 2026-09-17:
  - leak gate: 0 of 75 renameable, 0 of 37 sub-threshold, 0 separator-split,
    four controls green
  - beat-contamination audit: 0 of 3942 beats AND 0 of 3942 responses, against
    Hemingway's 70 of 7094 -- build_sft_pairs.py --source-entities earned its
    mandatory flag

Also records three provenance defects found on first read of the run and their
disposition: the hardcoded "r49-babyyarros-pairs-pilot" run label (cosmetic, same
literal on all three runs), the empty harness_commit (all three runs), and a
pairs_sha256_16 that is not a sha256sum of the file (consistent across runs, so a
cache key rather than a fault). The run is bound to McCarthy's pairs by record
count, not by the label.

scripts/mccarthy-corpus/voice-prep.py

@@ -0,0 +1,49 @@
+"""Adapt the lv-mccarthy arms for voice_distance.py, and refuse if its reference is empty.
+
+Same contract as the lv-hemingway sibling — the arm NAMES are the only real difference,
+and the control must carry the substring `unadapted` or voice_distance.py cannot identify
+it and prints an empty vs-control table instead of an error.
+
+voice_distance.py expects:
+  - files matching voice.<arm>.jsonl in the eval dir
+  - a `continuation` field per record (gen_beats_chat writes `raw`)
+  - a `seed` field (present)
+  - the control arm's NAME to contain the substring "unadapted"
+  - a reference built from corpus records whose split == "val"
+"""
+import json, sys
+from collections import Counter
+from pathlib import Path
+
+CORP = Path("/home/infra-ops/lv-mccarthy/corpus-renamed/copies")
+EVAL = Path("/home/infra-ops/r49-runs/mccarthy-eval")
+
+c = Counter()
+n = 0
+for p in sorted(CORP.glob("*.jsonl")):
+    for line in p.read_text(encoding="utf-8").splitlines():
+        if not line.strip():
+            continue
+        r = json.loads(line)
+        n += 1
+        c[r.get("split")] += 1
+print(f"corpus records: {n}  split values: {dict(c)}")
+if c.get("val", 0) == 0:
+    print("== REFUSING: no split=val records; voice_distance would build an EMPTY reference")
+    print("   and every delta_cb would be meaningless rather than absent.")
+    sys.exit(1)
+
+NAMES = {"base": "base-unadapted", "ckpt900": "ckpt900", "ckpt450": "ckpt450"}
+for src_arm, out_arm in NAMES.items():
+    src = EVAL / f"beats5.{src_arm}.jsonl"
+    if not src.exists():
+        print(f"== missing {src}")
+        sys.exit(1)
+    rows = [json.loads(l) for l in src.read_text(encoding="utf-8").splitlines() if l.strip()]
+    out = EVAL / f"voice.{out_arm}.jsonl"
+    with out.open("w", encoding="utf-8") as fh:
+        for r in rows:
+            fh.write(json.dumps({"id": r["id"], "seed": r["seed"],
+                                 "continuation": r["raw"]}, ensure_ascii=False) + "\n")
+    print(f"  {src.name} -> {out.name}  ({len(rows)} records)")
+print("ready")