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.

CHANGELOG.md

@@ -4,6 +4,50 @@ All notable changes to seismo-relay are documented here.
 
 ---
 
+## v0.7.0 — 2026-04-03
+
+### Added
+- **Raw ADC waveform decode — `_decode_a5_waveform(frames_data, event)`** in `client.py`.
+  Parses the complete set of SUB 5A A5 response frames into per-channel time-series:
+  - Reads the STRT record from A5[0] (bytes 7+): extracts `total_samples` (BE uint16 at +8),
+    `pretrig_samples` (BE uint16 at +16), and `rectime_seconds` (uint8 at +18) into
+    `event.total_samples / pretrig_samples / rectime_seconds`.
+  - Skips the 6-byte preamble (`00 00 ff ff ff ff`) that follows the 21-byte STRT header;
+    waveform data begins at `strt_pos + 27`.
+  - Strips the 8-byte per-frame counter header from A5[1–6, 8] before appending waveform bytes.
+  - Skips A5[7] (metadata-only) and A5[9] (terminator).
+  - **Cross-frame alignment correction**: accumulates `running_offset % 8` across all frames
+    and discards `(8 − align) % 8` leading bytes per frame to re-align to a T/V/L/M boundary.
+    Required because individual frame waveform payloads are not always multiples of 8 bytes.
+  - Decodes as 4-channel interleaved signed 16-bit LE at 8 bytes per sample-set:
+    bytes 0–1 = Tran, 2–3 = Vert, 4–5 = Long, 6–7 = Mic.
+  - Stores result in `event.raw_samples = {"Tran": [...], "Vert": [...], "Long": [...], "Mic": [...]}`.
+- **`download_waveform(event)` public method** on `MiniMateClient`.
+  Issues a full SUB 5A stream with `stop_after_metadata=False`, then calls
+  `_decode_a5_waveform()` to populate `event.raw_samples` and `event.total_samples /
+  pretrig_samples / rectime_seconds`. Previously only metadata frames were fetched during
+  `get_events()`; raw waveform data is now available on demand.
+- **`Event` model new fields** (`models.py`): `total_samples`, `pretrig_samples`,
+  `rectime_seconds` (from STRT record), and `_waveform_key` (4-byte key stored during
+  `get_events()` for later use by `download_waveform()`).
+
+### Protocol / Documentation
+- **SUB 5A A5[0] STRT record layout confirmed** (✅ 2026-04-03, 4-2-26 blast capture):
+  - STRT header is 21 bytes: `b"STRT"` + length fields + `total_samples` (BE uint16 at +8) +
+    `pretrig_samples` (BE uint16 at +16) + `rectime_seconds` (uint8 at +18).
+  - Followed by 6-byte preamble: `00 00 ff ff ff ff`. Waveform begins at `strt_pos + 27`.
+  - Confirmed: 4-2-26 blast → `total_samples=9306`, `pretrig_samples=298`, `rectime_seconds=70`.
+- **Blast/waveform mode A5 format confirmed** (✅ 2026-04-03, 4-2-26 blast capture):
+  4-channel interleaved int16 LE at 8 bytes per sample-set; cross-frame alignment correction
+  required. 948 of 9306 total sample-sets captured via `stop_after_metadata=True` (10 frames).
+- **Noise/histogram mode A5 format — endianness corrected** (✅ 2026-04-03, 3-31-26 capture):
+  32-byte block samples are signed 16-bit **little-endian** (previously documented as BE).
+  `0a 00` → LE int16 = 10 (correct noise floor); BE would give 2560 (wrong).
+- Protocol reference §7.6 rewritten — split into §7.6.1 (Blast/Waveform mode) and §7.6.2
+  (Noise/Histogram mode), each with confirmed field layouts and open questions noted.
+
+---
+
 ## v0.6.0 — 2026-04-02
 
 ### Added