fix: Address bugs in multiplayer-fps example

[krazyjakee]

Summary

Four small bugfixes uncovered while reviewing the examples/multiplayer-fps scripts:

• node-pool.gd — node.is_instance_valid() is invalid; is_instance_valid is a global function. Calling clear() would raise. Changed to is_instance_valid(node).
• player.gd — death sound, deaths counter, and die() were fired on every tick while health <= 0, so they multi-triggered between damage and respawn (worst on non-authority peers where die() early-returns). Now fires once on the alive → dead transition via a local _was_alive flag.
• player-input.gd — _notification(NOTIFICATION_WM_WINDOW_FOCUS_IN) ran on every player's Input node in the local tree, so window refocus called Input.set_mouse_mode(CAPTURED) once per remote player and cleared override_mouse on Inputs the local peer doesn't own. Now early-returns for non-authority Input nodes.
• bullethole.gd — replaced the hardcoded get_tree().root.get_node("multiplayer-fps/Map/StaticBody3D") lookup with an @export var spawn_root: Node, falling back to get_tree().current_scene. The example keeps working without scene edits and the component is reusable elsewhere.

Test plan

• Open examples/multiplayer-fps/multiplayer-fps.tscn and host a local server
• Connect a second client; confirm each player still spawns at a deterministic spawn point
• Shoot the other player to zero health; confirm the death sound plays exactly once per death and deaths increments by 1
• Alt-tab away from and back to the game window with two clients running; confirm only the local player's mouse is recaptured and remote Input nodes are unaffected
• Confirm bullet hole decals still spawn on the map geometry