P7.8: Port FPS controller to netfox BaseNetInput pattern

- FPSCharacterController: removed SimulationServer dependency, added
  _rollback_tick() with acceleration-based CharacterBody3D movement,
  gravity, jump, crouch/sprint speed. Kept mouse look, crouch lerp,
  sprint/crouch toggle, _move_local() standalone fallback.
- Player: extends FPSCharacterController (CharacterBody3D). Added
  authority-guarded _rollback_tick that delegates to super for server
  and local client prediction. TickInterpolator created dynamically
  (avoids headless class_name parse errors). PlayerNetInput child
  wired via existing RollbackSynchronizer input_properties.
- player.tscn: CharacterBody3D root with FpsCamera, CollisionShape3D,
  PlayerNetInput children. TickInterpolator created in code.
- client_main.gd: _spawn_local_player() creates FPS player node with
  first-person camera for own peer. _spawn_remote_player() removes
  FpsCamera for remote players.
- fps_camera.gd: duck-typed _controller (Node) instead of class_name
  cast for headless safety. Fixed type inference warnings.

Also includes parent task P7.5-P7.7 changes:
- project.godot: main_scene -> server_main.tscn
- network_manager.gd: Chan enum -> raw channel ints
- server_main.gd: deferred ServerConfig load, GameServer load()
- game_server.gd: commented out dev deps for headless compilation

project.godot

@@ -12,7 +12,7 @@ config_version=5
 
 config/name="Tactical Shooter"
 config/description="Phase 0 — Headless Dedicated Server"
-run/main_scene="res://scenes/entry.tscn"
+run/main_scene="res://scenes/server/server_main.tscn"
 config/features=PackedStringArray("4.7", "Forward Plus")
 
 [autoload]