Add Plantronics/Poly Voyager 8200 UC (BT600) support

[ggiesen]

Changes made

Adds support for the Plantronics/Poly Voyager 8200 UC headset via the BT600 USB dongle (047F:02EE), implementing sidetone, battery status, lights, voice prompts and volume limiter (CAP_SIDETONE | CAP_BATTERY_STATUS | CAP_LIGHTS | CAP_VOICE_PROMPTS | CAP_VOLUME_LIMITER).

Protocol. The dongle speaks Poly's native get/set protocol tunneled over HID report ID 0x07 in the vendor collection usage-page 0xFFA2 / usage 0x03 on interface 3. Commands are sent as OUTPUT reports (the interface has no interrupt-OUT endpoint, so they go out as SET_REPORT over EP0) and answered as INPUT reports on interrupt EP 0x81. 64-byte frames, zero padded:

07 01 01 10 <len> <addr> 00 00 <op> <grp> <item> <value...>

addr: 0x00 = the dongle, 0x20 = the headset (replies: 0x00 / 0x02)

HELLO (open session):  07 01 01 10 07 <addr> 00 00 01 01 02
HELLO ack:             07 01 01 10 06 <addr> 00 00 08 01 02
GET (read a setting):  07 01 01 10 06 20 00 00 02 GG II
SET (write a setting): 07 01 01 10 07 20 00 00 05 GG II VV
GET reply:             07 01 01 10 LL 02 00 00 03 GG II <value>
SET ack ("changed"):   07 01 01 10 LL 02 00 00 0a GG II VV

• The dongle ignores every GET/SET until sessions are opened. The required init (replayed from what the Windows stack sends at device attach, and verified live against a freshly power-cycled headset) is: a 2-byte 06 01 output report, a HELLO addressed to the dongle (this arms the relay to the headset), then a HELLO addressed to the headset. Sessions persist until the dongle loses power or the headset reboots; the sequence is idempotent and is sent before each operation.
• Sidetone: group 0x04 / item 0x10, 3 discrete levels (0=Low, 1=Medium, 2=High), mapped from the 0-128 input with the existing mapSidetoneToDiscrete<3>().
• Lights: group 0x04 / item 0x09 (the headset's "online indicator" LED, 0/1).
• Voice prompts: group 0x04 / item 0x11 (the "notification tones" setting, 0/1).
• Volume limiter: group 0x0F / item 0x0E (the "noise exposure" limit: 0=none, 1=85 dB, 2=80 dB). Mapped to 85 dB on / no limiting off, the same convention as the Lenovo Wireless VoIP implementation; the 80 dB middle value is documented but not exposed since the capability is boolean.
• All SETs are verified against the device's "changed" ack, so writing a setting with the headset absent reports device-offline instead of silently succeeding.
• Battery: group 0x0A / item 0x1A, value <level> <level_count> <charging> <minutes:u16be> 01. Percentage is level / (level_count - 1) * 100 -- Poly's own formula (its software computes level/numLevels*100), verified live against Poly Studio at 70% (level 7) and 60% (level 6). The remaining talk time (minutes) is a separate field reported as time_to_empty_min; it drifts within a level band, so it is not used for the percentage.
• With the headset powered off or out of range, nothing on this channel is answered (including the headset-addressed HELLO), so read timeouts are reported as device-offline.

Platforms. Linux and Windows. The Windows usagepage/usageid values were confirmed against the dongle's HID report descriptor (report 0x07 lives in the 0xFFA2/0x03 collection). macOS is left disabled: the dongle exposes five top-level HID collections and macOS opens only the first enumerated one, which I cannot test with this device; it can be enabled later if someone with a Mac confirms it works.

How it was derived. Reverse-engineered from USBPcap captures of Plantronics Hub and usbmon captures of Poly Studio, then validated live against the hardware on Linux (headset firmware v2120):

• Session HELLO verified on a cold dongle: before it, all requests are ignored; after it, they answer immediately.
• Battery percentage verified live against Poly Studio at two levels (level 7 = 70%, level 6 = 60%); charging flag verified in both directions on/off the charge cable.
• All three sidetone levels apply, are acked by the device, and read back correctly (cross-checked against the value set in Poly Studio).
• Lights, voice prompts and volume limiter were mapped by toggling each setting in Poly Studio while capturing (one SET per toggle), then validated live: each SET is acked and reads back with the new value.
• With the headset powered off, operations report device-offline.

Tests. Unit tests cover the battery reply parser (discharge/charging/clamping/malformed) and reply matching (SET acks, unsolicited events, post-HELLO catalogue fragments) using frames captured from the hardware, plus a registry lookup test. ctest passes; built with GCC 13 on Linux.

Notes.

• The BT600 dongle ships with several Poly headsets; this was only tested with the Voyager 8200 UC. Other Voyager models pairing with the same dongle will match this implementation and will likely work (the protocol is Poly-generic), but I cannot verify them.
• Poly software exposes more settings over the same group/item mechanism (mute alert type, ringtone selection, auto-answer, auto-pause, anti-startle, extended range mode, HD voice, streaming audio); these have been mapped from captures and are documented in the device header so the protocol knowledge is preserved, in case HeadsetControl grows matching capabilities.

Checklist

• I adjusted the README (if needed)
• For new features in HeadsetControl: I discussed it beforehand in Issues or Discussions and adhered to the wiki (N/A: adds device support only, no application changes)