AirPods Max support for linux-rust (encrypted AAP, battery, NC, on-head, head-tracking scaffold)

[Dino-dev66]

Summary

Adds working AirPods Max support to the linux-rust client, tested against a real AirPods Max (A2096, Lightning).

Root cause that unlocked everything

The L2CAP channel was opened unencrypted (BT_SECURITY defaulted to Low). AirPods accept writes over an unencrypted channel (so changing noise control appeared to work) but never emit the AAP notification stream — no battery, ear detection, mode readback, metadata or proximity keys. Setting the socket to Security::Medium before connect() (the equivalent of what proximity_keys.py/bumble do with authenticate() + encrypt()) makes the Max stream everything.

Now working on AirPods Max

• Battery monitoring — single Headphone (0x01) component, count=1
• Noise control — set + live readback (Off/Transparency/NC)
• On-head detection — drives media play/pause
• Connect/disconnect desktop notifications (notify-send, transient)

Head tracking / spatial audio

• Implemented the Pro-style 0x17 stream end to end: parse, start/stop senders, an AACPEvent::HeadTracking, a live pitch/yaw/roll visualiser, re-center, and a nod/shake GestureDetector mapped to media controls. This path is for models that support it (e.g. AirPods Pro).
• Gated off for AirPods Max (model A2096/A3184) with an in-app explanation: the Max exposes motion via a ProtectedAccess HID-over-AAP relay (cma / B2PRelayHIDDevice) that doesn't stream to an unauthenticated host. The Pro-style START packet only makes it re-advertise its HID descriptors.

Known hardware limits on the Max (not bugs)

• Conversational Awareness — the Max never advertises ConversationDetectConfig (H1 chip; Apple gates CA to H2).
• Digital Crown volume / play-pause are handled by the OS via AVRCP and send no AAP packets (nothing to wire app-side).

Notes

• No model gating existed before; the head-tracking UI is gated by the 0x1D model number.
• GNOME ignores libnotify expire_timeout, so notifications use the transient hint and are limited to connect/disconnect.

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