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/bimap.gd

@@ -0,0 +1,56 @@
+extends RefCounted
+class_name _BiMap
+
+# Maps one-to-one associations in a bidirectional way
+
+var _keys_to_values := {}
+var _values_to_keys := {}
+
+func put(key: Variant, value: Variant) -> void:
+	var old_value = _keys_to_values.get(key)
+	var old_key = _values_to_keys.get(value)
+
+	if old_value != null: _values_to_keys.erase(old_value)
+	if old_key != null: _keys_to_values.erase(old_key)
+
+	_keys_to_values[key] = value
+	_values_to_keys[value] = key
+
+func get_by_key(key: Variant) -> Variant:
+	return _keys_to_values.get(key)
+
+func get_by_value(value: Variant) -> Variant:
+	return _values_to_keys.get(value)
+
+func has_key(key: Variant) -> bool:
+	return _keys_to_values.has(key)
+
+func has_value(value: Variant) -> bool:
+	return _values_to_keys.has(value)
+
+func erase_key(key: Variant) -> bool:
+	if not has_key(key):
+		return false
+
+	var value = get_by_key(key)
+	_values_to_keys.erase(value)
+	_keys_to_values.erase(key)
+	return true
+
+func erase_value(value: Variant) -> bool:
+	if not has_value(value):
+		return false
+
+	var key = get_by_value(value)
+	_values_to_keys.erase(value)
+	_keys_to_values.erase(key)
+	return true
+
+func size() -> int:
+	return _keys_to_values.size()
+
+func keys() -> Array:
+	return _keys_to_values.keys()
+
+func values() -> Array:
+	return _values_to_keys.keys()