Add framing and versioning to the app-to-UHID event protocol

[guaje]

Hi @anasvhora284, this follow-up comes from strengthening UHID event decoding in #10.

Background

The current local-socket protocol sends an event type followed by a fixed-size payload. It has no frame length or protocol version. The decoder can identify a truncated event, but an extra or missing byte may misalign the stream and cause every following byte to be interpreted as the wrong event.

The socket is local and peer-verified, so this is mainly a reliability and compatibility concern rather than a request for network-level complexity.

Goal

Define a small framed and versioned event format so malformed or future events can be rejected or skipped without corrupting the rest of the stream.

Proposed scope

• Inventory the app-side writer and UHID-side reader for every current event.
• Define a compact header containing at least protocol version, event type, and payload length.
• Set a conservative maximum payload length before allocating or reading data.
• Validate exact payload lengths for known event types.
• Decide whether unknown event types can be skipped safely by length.
• Include shutdown behavior in the framed protocol design.
• Define compatibility and rollout behavior while the app and generated UHID DEX may be on different versions.
• Add deterministic codec tests for valid, truncated, oversized, unknown, and consecutive frames.
• Test that one malformed frame does not misalign the next valid frame where recovery is supported.
• Update the generated UHID DEX asset through the normal build task.

Non-goals

• Adding encryption or checksums without a demonstrated need on the peer-verified local socket.
• Depending on Android sockets or hardware in ordinary JVM tests.
• Mixing pointer sensitivity or scroll-speed settings into the wire-format change.

The format should remain intentionally small and should be agreed before implementation because both sides must migrate together.

[anasvhora284]

Closed as superseded by #44 (v1.4.2).

Context:

This issue originally tracked adding framing, versioning, and length prefixes to the local socket IPC between the Android app and the separate uhid-server Java DEX sidecar daemon.

Resolution:

PR #44 completely removed the uhid-server subproject, DEX asset, and local socket IPC. Input Leaf now interacts directly with the Linux kernel /dev/uhid character device in-process via Shizuku (UhidChannel.kt and UhidProtocol.kt) using standard Linux kernel uhid_event structs.

Because the standalone daemon and custom socket protocol no longer exist, framing and versioning for that protocol are no longer required.