AlertNotification Service/Client are not compliant to the spec

Verification

• I searched for similar bug reports (including closed issues) and found none was relevant.

What happened?

AlertNotification Service/Client use a 3 bytes header, the spec defines a 2 bytes header

What should happen instead?

AlertNotification Service/Client should comply to the BLE spec.

Reproduction steps

N/A

More details?

This issue was reported by the Amazfish community : InfiniTime expects the header of all notifications is a 3 bytes header (here and here).

However, the GATT spec specifies a header of 2 bytes.

Now, even if InfiniTime is not 100% compliant, all companion apps have currently implemented this 3 bytes header, and changing this might break the compatibility with those companion apps so I'm not sure if we should fix this.
If we do want to change the header size, we'll have to communicate with companion app developers to ensure smooth transition.

It might be possible to support both 2 and 3 bytes header by checking the value of the 3rd byte of the buffer : it can be ignored if it's equal to 0x00 or lower than 0x20.

The documentation should however be completed to mention that this is not completely compliant to the spec of the AlertNotificationService.

Version

<=1.13.0

Companion app

All

Start with doc/ble.md and compare its New Alert description with the linked BLE specification and the AlertNotificationService/Client behavior referenced in the issue. Resolve whether compatibility requires supporting both header sizes or retaining the current format, then document the resulting behavior and verify any implementation change against companion-app compatibility.