fix(paired-data): 4 bugs corrupting/blocking camera-supervised training data (#1007)

1. record-csi-udp.py stamped LOCAL time with a 'Z' (UTC) suffix → camera/CSI disagreed
   by the UTC offset → 0 aligned pairs. Now writes true UTC via datetime.now(timezone.utc).
2. align-ground-truth.js kept empty-keypoint (non-detection) records at confidence 0,
   collapsing window avgConf below threshold → all windows rejected. Now skipped at load.
3. extractCsiMatrix silently zero-padded/truncated mixed-subcarrier frames. Now frames
   are filtered to the session's modal subcarrier count before windowing — never padded.
4. CSI/feature matrices are filled frame-major but were labeled shape [nSc, nFrames] —
   transposed. Labels corrected to [nFrames, nSc] / [nFrames, dim].

Co-Authored-By: claude-flow <ruv@ruv.net>

scripts/record-csi-udp.py

@@ -15,6 +15,7 @@ import os
 import socket
 import struct
 import time
+from datetime import datetime, timezone
 
 
 def parse_csi_packet(data):
@@ -41,7 +42,8 @@ def parse_csi_packet(data):
 
     return {
         "type": "raw_csi",
-        "timestamp": time.strftime("%Y-%m-%dT%H:%M:%S.") + f"{int(time.time() * 1000) % 1000:03d}Z",
+        # true UTC, not local-time-labeled-Z (#1007 Bug 1) — e.g. "2026-06-17T01:23:45.678Z"
+        "timestamp": datetime.now(timezone.utc).isoformat(timespec="milliseconds").replace("+00:00", "Z"),
         "ts_ns": time.time_ns(),
         "node_id": node_id,
         "rssi": rssi,