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)

client/map_template/assets/prefabs/buy_zone.tscn

@@ -0,0 +1,28 @@
+[gd_scene load_steps=4 format=3]
+
+[sub_resource type="StandardMaterial3D" id="ZoneMat"]
+albedo_color = Color(0.8, 0.8, 0.2, 0.3)
+metallic = 0.0
+roughness = 0.5
+transparency = 0.7
+flags_transparent = true
+
+[sub_resource type="CapsuleShape3D" id="ZoneShape"]
+radius = 2.0
+height = 4.0
+
+[node name="BuyZone" type="Area3D"]
+groups = ["buy_zone"]
+; Buy zone — players inside this Area3D can purchase weapons/equipment.
+; Resize the CollisionShape3D child to match the room.
+; Monitor player bodies with area_entered / area_exited signals.
+
+[node name="CollisionShape3D" type="CollisionShape3D" parent="."]
+shape = SubResource("ZoneShape")
+
+[node name="Visualizer" type="CSGBox3D" parent="."]
+size = Vector3(4.0, 3.0, 4.0)
+material = SubResource("ZoneMat")
+use_collision = false
+; Semi-transparent yellow box to visualise the zone in the editor.
+; Disabled in-game — the game uses the Area3D collision, not this visualiser.