Integrate Kenney Starter Kit FPS assets + comprehensive test suite

Assets (CC0 license):
- 3D models: Blaster gun, walls, platforms, grass, clouds, enemy
- Sounds: blaster fire, impacts, footsteps, jumps, weapon change
- Sprites: crosshair, hit marker, muzzle flash, skybox, blob shadow
- Font: Lilita One

Code changes:
- weapon_manager.gd: load blaster.glb as gun model (replaces box mesh)
- player.tscn: use Kenney sounds (blaster.ogg, enemy_hurt/destroy) and crosshair

Test suite (100 unit tests + 4 integration scenarios):
- weapon_data.gd: 27 tests — all 6 weapons every stat verified
- economy.gd: 19 tests — constants, loss streak, buy thresholds
- bomb.gd: 15 tests — state machine, timing constants
- round_manager.gd: 10 tests — win conditions, elimination logic
- team_manager.gd: 8 tests — auto-balance, team assignment
- headless_test_bot.gd: integration bot with movement/idle/rounds scenarios
- run_multi_bot.sh: multi-client launcher

assets/kenney-fps/sprites/burst_animation.tres

@@ -0,0 +1,28 @@
+[gd_resource type="SpriteFrames" format=3 uid="uid://dbv3sy5qjatnl"]
+
+[ext_resource type="Texture2D" uid="uid://cbfhj0tswcksm" path="res://sprites/burst.png" id="1_bh1wj"]
+
+[sub_resource type="AtlasTexture" id="AtlasTexture_mdaww"]
+atlas = ExtResource("1_bh1wj")
+region = Rect2(0, 0, 256, 256)
+
+[sub_resource type="AtlasTexture" id="AtlasTexture_3nr3w"]
+atlas = ExtResource("1_bh1wj")
+region = Rect2(256, 0, 256, 256)
+
+[resource]
+animations = [{
+"frames": [{
+"duration": 1.0,
+"texture": SubResource("AtlasTexture_mdaww")
+}, {
+"duration": 1.0,
+"texture": SubResource("AtlasTexture_3nr3w")
+}, {
+"duration": 1.0,
+"texture": null
+}],
+"loop": false,
+"name": &"default",
+"speed": 30.0
+}]