fix: restore live-decode regressions guards

lib/src/live.dart

@@ -314,8 +314,9 @@ _Motion? _r10Motion(ByteData view, int len) {
   // (verified on real captures — consecutive R10 timestamps step by exactly
   // 1 s while each record carries 100 samples per axis). At that rate:
   //
-  //   • the detrend window below is ±9 samples = 190 ms, which high-passes
-  //     around 5 Hz and attenuates the 1.5–3 Hz gait fundamental before the
+  //   • the detrend window below is ±9 samples = 190 ms. Subtracting a 19-tap
+  //     moving average high-passes at about 0.443·fs/N ≈ 2.3 Hz, which sits
+  //     inside the 1.5–3 Hz gait fundamental and attenuates it before the
   //     search ever runs;
   //   • the autocorrelation lag window (7..40 samples = 70–400 ms) can only
   //     resolve cadences of 150–857 steps/min. Ordinary walking is 100–130
@@ -426,6 +427,25 @@ DecodedSample? decodeRecord(String hex) {
     return DecodedSample(
       ts: d.tsEpoch,
       hr: d.hr,
+      // On `wristOn` below: skinContact is contact QUALITY, not wear. 28 real
+      // v24 records in the parity fixture read hr 87-97 (clearly worn) with
+      // skinContact <= 50, so a contact threshold would call them off-wrist.
+      // `hr > 0` is kept for that reason.
+      //
+      // But the parity oracle cannot EVIDENCE it. `decode_parity_cases.json`'s
+      // `wrist_on` column was GENERATED by the same rule, verbatim:
+      //
+      //     wrist_on: d.hr > 0
+      //
+      // so the oracle agreeing is the expression read back, not confirmation.
+      // That line lived in `ts/live.ts`, which this commit removes; it is
+      // recorded here rather than by path so the provenance survives the
+      // deletion, and remains recoverable at protocol main 7edcb3e:
+      //
+      //     git show 7edcb3e:ts/live.ts | sed -n '229p'
+      //
+      // Confirming `hr > 0` needs captures with independently known wear
+      // state, not the fixture.
       // This record family carries no IMU stepping window at all — that is
       // "no usable IMU data", the same absence [DecodedSample.activity]'s
       // doc comment describes for a truncated R10, not a measured 0.0/0.