[XAP] finalise the HID protocol packet format

[tzarc]

XAP Task Info

Original Issue
Original spec document
Current XAP Definitions
Current XAP Generated Docs
Placeholder PR

Description

Any work in this area needs to be discussed with QMK Collaborators first, and in this case primarily with @tzarc due to their prior work in this area.

• The XAP protocol when communicating over HID only works with fixed-length packets.
• The original specification defines a request/response pattern, with a correlation token in order to determine when a response is received.
• The request/response pattern is not fully synchronous, in the sense that there is no guarantee that the next received packet is the corresponding response
• The request/response pattern is somewhat synchronous, in the sense that there is a guarantee that there is only one "in-flight" request being handled by the firmware at any one time, and as such the received responses will be in the same order as the original requests (interspersed with other packets as required)
• Broadcast messages (like logging output) may be received before the actual response is received by the host app
• Host apps will not receive copies of any outbound packets made by other host apps
• Host apps will receive all responses/broadcasts regardless of whether or not they were the initiator of the request
• Need to decide if the packet format is capable of catering for all possible usages
• Need to decide if a randomly-generated 16-bit token is enough, or if it should be bumped to 24- or 32-bit
• Need to investigate whether or not feature reports should be used instead

[stale]

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