fix(scratch): dedupe the DL2 Sent/ mirror; correct the recovered-file count

The DL2 export keeps a byte-identical `Sent/` copy of its root, so walking it
counts every binary twice: 127,035 histogram paths are 63,535 distinct files,
and 13,077 waveform paths are 6,577. offset_scan.py now keeps the first
occurrence of each basename.

Corrects the previous commit's changelog claim of 8 recovered files — it is 4:
K440HJCN.3C0H and K557IF1U.8K0H (stride 252), T191HVNP.0S0H (92), T193L0XM.CI0H
(612). Still zero regressions. The per-unit breakdown reading exactly 2-2-2-2
should have given the doubling away.

The 14,338-exact verification result is unaffected: ASCII exports are not
mirrored (14,340 paths, 14,340 distinct names), and the harness enumerates
those rather than the binaries.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HgTe8CamXAHcAmaQ6QNcog

scratch/offset_scan.py

@@ -107,8 +107,15 @@ def main():
     ap.add_argument("--out", required=True)
     a = ap.parse_args()
 
-    files = [p for p in Path(a.dir).rglob("*") if p.is_file() and _WAVE_RE.search(p.name)]
-    files.sort()
+    # The DL2 export keeps a byte-identical `Sent/` mirror of the root, so
+    # enumerate paths but keep only the first occurrence of each basename —
+    # otherwise every event is counted twice.
+    seen = set()
+    files = []
+    for q in sorted(Path(a.dir).rglob("*")):
+        if q.is_file() and _WAVE_RE.search(q.name) and q.name not in seen:
+            seen.add(q.name)
+            files.append(q)
     if a.limit:
         files = files[: a.limit]
     print(f"waveform binaries to scan: {len(files)}", flush=True)