Harden Keychron Nape Pro tests and raise model DPI ceiling to 4000

[dwei30]

Summary

• Extract keychronDecodeBattery into the Keychron protocol layer and use it from KeychronHidClient.
• Add codec and fake-HID driver tests for packet framing, status reads, DPI/polling write-back, and Link-KM rejection.
• Raise the Nape Pro DPI ladder to 50–4000 (step 50) via KEYCHRON_NAPE_DPI_* so future Keychron models can define their own ceilings.

Hardware verification

Keychron Nape Pro firmware v1.2.6-ZK Jun 30 2026 09:17:58:

• Wired0x3434:0x0440 — status read, DPI write (1600), polling write (1000 Hz)
• Link-KM0x3434:0xd026 — status read, DPI write (3200 / 800), polling write (125 Hz)
• Both paths advertised polling rates 125 / 500 / 1000 Hz only on this firmware
• Stage 5 was observed storing 4000 DPI, which was changed.

Notes

• Matching OpenMouse PR: Add Keychron Nape Pro preview, device art, and activate open openmouse#63

Test plan

• npm run check
• Wired Nape Pro: read status, set DPI, set polling
• Link-KM: read status, set DPI, set polling
• Confirm UI offers up to 4000 DPI and accepts a 4000 write
• Confirm unrelated Link-KM pairings still reject cleanly