feat: implement raw ADC waveform decoding and download functionality

- Added `_decode_a5_waveform()` to parse SUB 5A frames into per-channel time-series data.
- Introduced `download_waveform(event)` method in `MiniMateClient` to fetch full waveform data.
- Updated `Event` model to include new fields: `total_samples`, `pretrig_samples`, `rectime_seconds`, and `_waveform_key`.
- Enhanced documentation in `CHANGELOG.md` and `instantel_protocol_reference.md` to reflect new features and confirmed protocol details.

minimateplus/models.py

@@ -327,12 +327,19 @@ class Event:
     # Raw ADC samples keyed by channel label.  Not fetched unless explicitly
     # requested (large data transfer — up to several MB per event).
     raw_samples: Optional[dict] = None      # {"Tran": [...], "Vert": [...], ...}
+    total_samples: Optional[int] = None     # from STRT record: expected total sample-sets
+    pretrig_samples: Optional[int] = None   # from STRT record: pre-trigger sample count
+    rectime_seconds: Optional[int] = None   # from STRT record: record duration (seconds)
 
     # ── 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)
 
+    # 4-byte waveform key used to request this event via SUB 5A.
+    # Set by get_events(); required by download_waveform().
+    _waveform_key: Optional[bytes] = field(default=None, repr=False)
+
     def __str__(self) -> str:
         ts = str(self.timestamp) if self.timestamp else "no timestamp"
         ppv = ""