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)

addons/netfox.internals/set.gd

@@ -0,0 +1,64 @@
+extends RefCounted
+class_name _Set
+
+var _data: Dictionary = {}
+var _iterator_idx: int = -1
+
+static func of(items: Array) -> _Set:
+	var result := _Set.new()
+	for item in items:
+		result.add(item)
+	return result
+
+func add(value):
+	_data[value] = true
+
+func has(value) -> bool:
+	return _data.has(value)
+
+func size() -> int:
+	return _data.size()
+
+func is_empty() -> bool:
+	return _data.is_empty()
+
+func erase(value):
+	return _data.erase(value)
+
+func clear():
+	_data.clear()
+
+func values() -> Array:
+	return _data.keys()
+
+func min():
+	return _data.keys().min()
+
+func max():
+	return _data.keys().max()
+
+func equals(other) -> bool:
+	if not other or not other is _Set:
+		return false
+
+	return values() == other.values()
+
+func _to_string():
+	return "Set" + str(values())
+
+func _iter_init(arg) -> bool:
+	_iterator_idx = 0
+	return _can_iterate()
+
+func _iter_next(arg) -> bool:
+	_iterator_idx += 1
+	return _can_iterate()
+
+func _iter_get(arg):
+	return _data.keys()[_iterator_idx]
+
+func _can_iterate() -> bool:
+	if _data.is_empty() or _iterator_idx >= _data.size():
+		_iterator_idx = -1
+		return false
+	return true