fix: serial decode offset, PPV label scan, debug mode for waveform records

- _decode_serial_number: read from data[11:] not data[:8] — was returning
  the LENGTH_ECHO byte (0x0A = '\n') instead of the serial string
- _extract_peak_floats: search for channel label strings ("Tran" etc) and
  read float at label+6; old step-4 aligned scan was reading trigger levels
  instead of PPV values
- get_events: add debug=False param; stashes raw 210-byte record on
  Event._raw_record when True for field-layout inspection
- server /device/events: add ?debug=true query param; includes
  raw_record_hex + raw_record_len in response when set
- models: add Event._raw_record optional bytes field

minimateplus/models.py

@@ -197,6 +197,11 @@ class Event:
     # requested (large data transfer — up to several MB per event).
     raw_samples: Optional[dict] = None      # {"Tran": [...], "Vert": [...], ...}
 
+    # ── Debug / introspection ─────────────────────────────────────────────────
+    # Raw 210-byte waveform record bytes, set when debug mode is active.
+    # Exposed by the SFM server via ?debug=true so field layouts can be verified.
+    _raw_record: Optional[bytes] = field(default=None, repr=False)
+
     def __str__(self) -> str:
         ts = str(self.timestamp) if self.timestamp else "no timestamp"
         ppv = ""