fix: remove special case chunk counter, all chunks use chunk_num * 0x0400

CLAUDE.md

@@ -109,6 +109,22 @@ S3→BW (response):
 Both differences confirmed by reproducing Blastware's exact wire bytes from the 1-2-26
 BW TX capture. All 10 frames verified.
 
+### SUB 5A — chunk counter is monotonic (CORRECTED 2026-04-06)
+
+**Chunk counters are `chunk_num * 0x0400` for ALL chunks including chunk 1.**
+
+The 4-2-26 BW TX capture showed `counter=0x1004` for chunk 1 of event key `01110000`, which
+led to `_CHUNK1_COUNTER = 0x1004` being hardcoded as a special case. This was a Blastware
+artifact, not a protocol requirement. Empirical test 2026-04-06: with `counter=0x1004` for
+chunk 1 the device times out (120 s); with `counter=0x0400` (= `1 * 0x0400`) it responds
+immediately and streams all frames correctly.
+
+The 4-3-26 capture confirms the pattern for a second event (key `0111245a`):
+chunk 1 = `0x245A`, chunk 2 = `0x285A`, chunk 3 = `0x2C5A` (each +0x0400). Blastware's
+true formula is `key4[2:4] + n * 0x0400` — but since `key4[2:4]` of the first event is
+`0x0000`, `n * 0x0400` produces the right result. The device does not strictly validate the
+counter and streams data for any valid 5A request; using `chunk_num * 0x0400` is correct.
+
 ### SUB 5A — params are 11 bytes for chunk frames, 10 for termination
 
 `bulk_waveform_params()` returns 11 bytes (extra trailing `0x00`). The 11th byte was