Phase 7: netfox + godot-jolt stack upgrade

Stack installed:
- netfox v1.35.3 (core + extras + noray + internals)
- godot-jolt v0.16.0-stable

Architecture:
- Server: ENet transport (works headless, no netfox deps)
- Client/Editor: netfox rollback (RollbackSynchronizer, TickInterpolator)

New/modified:
- docs/migration-netfox-plan.md — migration architecture
- scripts/network/network_manager.gd — netfox-aware ENet fallback
- scripts/network/player.gd — clean base player
- client/characters/player_netfox.gd — rollback player w/ WeaponManager
- client/characters/input/player_net_input.gd — BaseNetInput subclass
- client/characters/character/fps_character_controller.gd — netfox input feed
- client/weapons/ — weapon data, registry, TacticalWeaponHitscan, WeaponManager
- client/scripts/round_replicator.gd — client-side round state bridge
- server/scripts/round_manager.gd — improved state machine
- server/scripts/plugin_api/plugin_manager.gd — refined plugin system
- config: enemy_tag, ally_tag for meatball targeting

Removed: old C++ SimulationServer GDExtension (replaced by netfox rollback)

tests/test_singleton.gd

@@ -0,0 +1,16 @@
+#!/usr/bin/env godot --headless -s
+extends SceneTree
+
+func _init():
+	print("=== Singleton Test ===")
+	var names = Engine.get_singleton_list()
+	for n in names:
+		print("  singleton: ", n)
+	
+	print("---")
+	print("RoundManager:", Engine.has_singleton("RoundManager"))
+	print("ServerConfig:", Engine.has_singleton("ServerConfig"))
+	print("NetworkManager:", Engine.has_singleton("NetworkManager"))
+	print("RoundManager singleton only:", Engine.has_singleton("RoundManager"))
+	
+	quit()