Feature: Full-Duplex Wired Split (#2766)

refactor(split): Refactor split code for extension

Extract central/peripheral code to allow for plugging in alternate
transports, instead of tying all split logic to BT.

feat(split): Add full-duplex wired split support

* Depends on full-duplex hardware UART for communication.
* Supports all existing central commands/peripheral events, including
  sensors/inputs from peripherals.
* Only one wired split peripheral supported (for now)
* Relies on chosen `zmk,split-uart` referencing the UART device.

docs: Add wired split config docs.

Migrate split to its own dedicated config file, and add details
on wired split config.

Co-authored-by: Nicolas Munnich <98408764+Nick-Munnich@users.noreply.github.com>

fix: Properly override stack size on RP2040

Move the system work queue stack size override on RP2040 ouf of
a `ZMK_BLE` conditional so it is properly applied generally for that
SoC.

---------

Co-authored-by: Nicolas Munnich <98408764+Nick-Munnich@users.noreply.github.com>

app/dts/bindings/zmk,wired-split.yaml

@@ -0,0 +1,21 @@
+# Copyright (c) 2025 The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+description: |
+  Complete specification of wired split connection
+
+compatible: "zmk,wired-split"
+
+properties:
+  device:
+    type: phandle
+    required: true
+    description: The UART device for wired split communication
+
+  half-duplex:
+    type: boolean
+    description: "Experimental: Enable half-duplex protocol mode"
+
+  dir-gpios:
+    type: phandle-array
+    description: "Experimental: Set the communication direction. Used for RS-422 style comms."