AdvancedParaEQ: correct V2 band-stride and header length

The V2 parser was using stride [filter, gain_hi, gain_lo, freq_hi,
freq_lo] with a 3-byte header. Pcap traces of LGHUB ↔ G522 LIGHTSPEED
show the actual wire is [freq_hi, freq_lo, filter, gain_hi, gain_lo]
with a 1-byte header (just dir_echo; the centurion bridge omits the
slot_echo the protocol spec describes for getCustomEQ).

With the old stride, slot 0's 10-band response on G522 parsed as 9
bands: the highest band (19 kHz, +2.75 dB on the active preset) was
silently swallowed because byte alignment landed on what looked like a
freq=0 end-sentinel. The displayed bands were also shifted — 20 Hz was
missing from the front because the parser started one full band late.

See ~/ghub/solaar_0x020D_slot_investigation.md for the RE notes.

lib/logitech_receiver/advanced_para_eq.py

@@ -23,16 +23,20 @@ V0/V1 wire format: 3-byte band stride [freq_hi, freq_lo, gain_i8],
 gain is whole dB; getEQInfos returns 5 bytes [bandCount, dbRange,
 caps, dbMin, dbMax].
 
-V2 wire format: 3-byte header [direction_echo, slot_echo,
-band_count_max], then N × 5-byte band stride [filter_type, gain_hi,
-gain_lo, freq_hi, freq_lo], then 0..2 trailer bytes (opaque, ignored).
-band_count_max is the device's max-bands capacity, NOT how many bands
-are populated — the parser consumes 5-byte chunks until <5 bytes
-remain. Frequency u16 BE in Hz, with freq=0 marking end-of-bands.
-Gain is **offset-binary**: raw 0..(steps-1) maps linearly to
-gain_min..gain_max (so on G522 with steps=241 / gain=[-6..6], raw=120
-= 0 dB). getEQInfos returns 13 bytes with gain bounds + step count,
-format enum, XY-support flag, and onboard preset counts.
+V2 wire format: 1-byte header [direction_echo], then N × 5-byte band
+stride [freq_hi, freq_lo, filter_type, gain_hi, gain_lo], with 0..3
+trailer bytes (opaque, ignored). The parser consumes 5-byte chunks
+until <5 bytes remain or a freq=0 sentinel is hit. Frequency u16 BE
+in Hz; gain u16 BE in **offset-binary**: raw 0..(steps-1) maps
+linearly to gain_min..gain_max (so on G522 with steps=241 /
+gain=[-6..6], raw=120 = 0 dB). getEQInfos returns 13 bytes with gain
+bounds + step count, format enum, XY-support flag, and onboard preset
+counts.
+
+(The protocol spec lists a 2-byte header [dir_echo, slot_echo] for
+getCustomEQ, but G522 firmware via the centurion bridge omits the
+slot_echo and emits a 1-byte header that matches getEQDefaults.
+Verified against pcap traces of LGHUB ↔ G522 LIGHTSPEED traffic.)
 """
 
 from __future__ import annotations
@@ -178,9 +182,9 @@ def parse_v2_bands(result: bytes, info: dict | None):
     """Parse a V2 getCustomEQ / getEQDefaults response.
 
     Wire layout (see module docstring):
-      [direction_echo, slot_echo, band_count_max]    (3-byte header)
-      N × [filter_type, gain_hi, gain_lo, freq_hi, freq_lo]   (5 bytes)
-      [trailer …]                                     (0..2 bytes, ignored)
+      [direction_echo]                              (1-byte header)
+      N × [freq_hi, freq_lo, filter_type, gain_hi, gain_lo]   (5 bytes)
+      [trailer …]                                   (0..3 bytes, ignored)
 
     Gain is offset-binary against `info`'s gain bounds:
       gain_db = gain_min + (gain_max - gain_min) * raw / (steps - 1)
@@ -194,9 +198,9 @@ def parse_v2_bands(result: bytes, info: dict | None):
     valid header returns []. Bands with freq=0 are treated as the
     end-of-bands sentinel (matches V0/V1 behavior at lines below).
     """
-    if result is None or len(result) < 3:
+    if result is None or len(result) < 1:
         return None
-    payload = result[3:]  # skip [dir_echo, slot_echo, band_count_max]
+    payload = result[1:]  # skip [dir_echo]
     band_size = 5
     if info:
         gain_min = info.get("gain_min_db", -6)
@@ -207,9 +211,9 @@ def parse_v2_bands(result: bytes, info: dict | None):
     bands = []
     for i in range(len(payload) // band_size):
         e = payload[i * band_size : (i + 1) * band_size]
-        filter_type = e[0]
-        gain_raw = (e[1] << 8) | e[2]
-        freq_hz = (e[3] << 8) | e[4]
+        freq_hz = (e[0] << 8) | e[1]
+        filter_type = e[2]
+        gain_raw = (e[3] << 8) | e[4]
         if freq_hz == 0:
             break  # disabled band — end-of-bands sentinel
         if steps > 1:
@@ -255,10 +259,6 @@ def get_advanced_eq_defaults(device, direction=DIRECTION_PLAYBACK, slot=0):
                 result.hex(),
             )
             return None
-        # Log raw=... too — getEQDefaults appears to use a different header
-        # framing than getCustomEQ on G522 (decoded values come out shifted
-        # by a byte). Capture the raw bytes so we can pin down the actual
-        # layout difference and adjust the parser accordingly.
         logger.info(
             "AdvancedParaEQ getEQDefaults V2 (dir=%d slot=%d): %d band(s) raw=%s %s",
             direction,