feat(keychron): add M6 wired and receiver support

[ydw1904]

Summary

• Adds a dedicated Keychron M6 WebHID driver for wired USB (3434:d060) and the paired 2.4 GHz Keychron Link-KM receiver (3434:d029).
• Limits receiver support to its verified ffc1:01 control collection, so the existing Nape Pro raw-HID receiver path remains separate.
• Supports status reads plus DPI and polling-rate writes with read-back verification.

Scope

• Bluetooth is intentionally outside this WebHID driver. Keychron documents Launcher configuration through a cable or 2.4 GHz receiver, not Bluetooth: https://www.keychron.com/blogs/news/how-to-set-pointer-and-report-rate-on-launcher

Hardware validation

• Verified a physical Keychron M6 in wired USB mode and through the Link-KM 2.4 GHz receiver.
• Both paths use status b3/b4 and settings b5/b6 reports on ffc1:01.
• On each path, DPI stage 0 changed 400 -> 1200 -> 400 and read back after each step.
• On each path, polling changed 500 Hz -> 1000 Hz -> 500 Hz and read back after each step.
• The mouse finished each test at its original 400 DPI and 500 Hz settings.

Tests

• TypeScript build passes.
• Full suite: 525 passing tests.
• Package contents were inspected after the build. The repository's npm check wrapper could not run because this environment does not provide npm; its build and test steps were run directly.

Assisted by Codex.

[ydw1904]

Companion PR in the web app: OpenMouse-Project/openmouse#128 (draft, blocked on this one).

Worth flagging because this PR alone does not make the M6 usable. KeychronM6HidClient is detected fine — the registry auto-detects it from the report descriptor — but the app's pulsarClient() in src/device/controller.ts is a negative check: any active client not listed in DEDICATED is assumed to be Pulsar and routed into showPulsarExplorer(), which calls the Pulsar-only describeCollections(). The device lists in the sidebar as "Keychron · Available" and activation fails with client.describeCollections is not a function.

#128 adds one entry to NEEDS_OPEN to fix it. It is a draft only because it imports @openmouse/protocol/drivers/keychron/m6-hid, so its type check cannot pass until this merges and the protocol pin moves.

Verified on wired hardware with both PRs applied: connects, reports 800 DPI / 500 Hz / battery, and a DPI change flashes and reads back.