docs: specify the rooted Android head-tracker bridge

[IvanChanPing]

Summary

• defines the AOSP head-tracker HID v1 reports and Bluetooth route identity
• maps the /dev/uhid create, feature-report, input-report, and teardown lifecycle
• records root/OEM spatializer preconditions and the unresolved AirPods-axis verification gate

This is an implementation-contract draft for the system route in #719, not a completed UHID bridge. It stays within LibrePods' stated scope by excluding an app-owned stereo renderer.

Verification

• cross-checked against the current AOSP head-tracker HID specification and Linux UHID ABI
• cross-checked against LibrePods Android and PR AirPods Max support for linux-rust (encrypted AAP, battery, NC, on-head, head-tracking scaffold) #617 head-tracking parsers
• documentation diff checked locally

Also carries the system-delivery portion of the head-tracking research in #713.