Dispatch Brick Fn keys with a spruce-native watchdog

[Felixmil]

On the TrimUI Brick, the "Fn Key and Switch Settings" app lets you map an action to each of the two physical Fn keys, but pressing them did nothing. This adds Fn key support the spruce-native way: a small watchdog reads the Fn key presses and runs the mapped action, with no stock keymon.

This supersedes the earlier approach in #1548, which enabled Fn keys by adding the stock keymon daemon to the boot blob list. That PR was closed because spruce deliberately does not run keymon (it was removed across the project and replaced with spruce's own getevent watchdogs; keymon is kept only for the Miyoo Mini, where it is a different, stock component). This PR delivers the same feature using the existing watchdog pattern instead, so it fits that decision.

Fn key watchdog

spruce/brick/fnkey_watchdog.sh reads Fn key-down events from $EVENT_PATH_READ_INPUTS_SPRUCE with getevent (the same idiom as homebutton_watchdog.sh) and runs the action the fneditor app mapped to each key. The chosen action is read fresh on every press from /usr/trimui/fnkeys/f1key_launch (left) and f2key_launch (right), so remapping in the editor takes effect immediately, and an unmapped key is ignored. The action scripts are stateless togglers invoked with no argument, so the watchdog just executes the launch path, exactly as keymon did.

On the Brick the left/right Fn keys report as $B_L3 (1 317) and $B_R3 (1 318) on event3; the Brick has no analog sticks, so those codes are exclusively the Fn keys. The node is read non-exclusively, alongside the home and buttons watchdogs that already read it, so there is no double-handling.

Boot wiring

launch_startup_watchdogs in trimui_a133p.sh launches and CPU-pins the watchdog next to the common startup watchdogs, matching how the other durable watchdogs are started. The boot blob list is unchanged (no keymon).

Validation

Tested on a physical Brick (FW 1.1.1) from a clean reboot: keymon is not running, fnkey_watchdog.sh starts and is pinned, and it coexists with the home, buttons, power, and volume watchdogs on the shared event3 node. The right Fn key (mapped to the LCD brightness cycle) works end to end through spruce's own backlight control. The left Fn key's LED toggle dispatches correctly, but the stock LED action script is a no-op under spruce (it writes /tmp/system/enable_led, which the stock firmware daemons consume and spruce does not); making the physical RGB respond is a separate, pre-existing gap in that action script and is left for a follow-up.

Closes #1440

[Felixmil]

Heads-up on ordering: PR #1549 (Brick Fn action + volume-key sync) is stacked on top of this one and should merge after it. That PR reuses the Fn key watchdog added here to dispatch its LED/backlight Fn actions, and moves its Brick-only watchdog launch into the same launch_startup_watchdogs in Brick.sh that this PR introduces.