t_p7.4_port: network_manager to netfox

- Fixed double-emission bug in network_manager.gd: ENet signal connections
  (peer_connected/peer_disconnected) are now only connected when netfox
  NetworkEvents is NOT available, preventing duplicate player_connected/
  player_disconnected signal emissions.
- Fixed stop() to only disconnect ENet signals when they were actually
  connected (mirrors the conditional connection in start_server()).
- Fixed headless parse error in player_net_input.gd: replaced direct
  NetworkTime.before_tick_loop reference with Engine.get_singleton()
  call, avoiding unresolved type identifier in headless mode.
- netfox_bootstrap.gd added as minimal autoload placeholder.
- Dual-path architecture: netfox NetworkEvents when available (editor),
  ENet fallback when netfox unavailable (headless server).
- Broadcast RPCs (spawn, round state, scores) kept as-is since they
  work identically through Godot's MultiplayerAPI in both paths.

scripts/network/netfox_bootstrap.gd

@@ -0,0 +1,10 @@
+## NetfoxBootstrap — minimal. All class_names should be registered by
+## Godot's global script scanning. We don't reference any netfox
+## class_name directly in our code.
+
+extends Node
+
+func _ready() -> void:
+	# netfox singletons are registered by the editor plugin only.
+	# In headless/export builds, Engine.has_singleton checks handle this.
+	pass