Add a main menu scene (#47)

MainMenu.tscn is now project.godot's entry point: Play / Map Editor
(disabled, explains itself - #48 wires it up) / Quit (hidden in web
builds), styled like the rest of the game's menus and reusing the
starfield shader for its background.

Play calls GameState.reset_new_game(), so a second playthrough in the
same process starts fresh instead of inheriting whatever the previous
run left in the autoload - existing tests still load Main.tscn
directly and are unaffected.

Part of the #40 conquest-mechanic breakdown.

MainMenu.tscn

@@ -0,0 +1,139 @@
+[gd_scene load_steps=4 format=3]
+
+[ext_resource type="Script" path="res://MainMenu.gd" id="1"]
+[ext_resource type="Script" path="res://IconButton.gd" id="2"]
+[ext_resource type="Shader" path="res://starfield.gdshader" id="3"]
+
+[sub_resource type="ShaderMaterial" id="1"]
+shader = ExtResource("3")
+shader_parameter/camera_cell = Vector2(0, 0)
+shader_parameter/camera_frac = Vector2(0, 0)
+shader_parameter/camera_zoom = 1.0
+shader_parameter/viewport_size = Vector2(720, 1280)
+shader_parameter/cell_size = 60.0
+shader_parameter/bg_color = Color(0.02, 0.02, 0.08, 1)
+
+[node name="MainMenu" type="Node2D"]
+script = ExtResource("1")
+
+[node name="BackgroundLayer" type="CanvasLayer" parent="."]
+layer = -10
+
+[node name="Starfield" type="ColorRect" parent="BackgroundLayer"]
+material = SubResource("1")
+mouse_filter = 2
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+grow_horizontal = 2
+grow_vertical = 2
+
+[node name="UI" type="CanvasLayer" parent="."]
+
+[node name="TitleLabel" type="Label" parent="UI"]
+anchor_left = 0.5
+anchor_right = 0.5
+anchor_top = 0.5
+anchor_bottom = 0.5
+offset_left = -300
+offset_right = 300
+offset_top = -560
+offset_bottom = -460
+theme_override_font_sizes/font_size = 60
+horizontal_alignment = 1
+text = "SPACESHIP TEST"
+
+[node name="PlayButton" type="Control" parent="UI"]
+anchor_left = 0.5
+anchor_right = 0.5
+anchor_top = 0.5
+anchor_bottom = 0.5
+offset_left = -64
+offset_right = 64
+offset_top = -300
+offset_bottom = -172
+script = ExtResource("2")
+icon = 2
+glow_color = Color(0.35, 0.95, 0.55, 1)
+
+[node name="PlayLabel" type="Label" parent="UI"]
+mouse_filter = 2
+anchor_left = 0.5
+anchor_right = 0.5
+anchor_top = 0.5
+anchor_bottom = 0.5
+offset_left = -160
+offset_right = 160
+offset_top = -160
+offset_bottom = -94
+theme_override_font_sizes/font_size = 32
+horizontal_alignment = 1
+text = "Play"
+
+[node name="EditorButton" type="Control" parent="UI"]
+anchor_left = 0.5
+anchor_right = 0.5
+anchor_top = 0.5
+anchor_bottom = 0.5
+offset_left = -64
+offset_right = 64
+offset_top = -20
+offset_bottom = 108
+script = ExtResource("2")
+icon = 0
+glow_color = Color(0.5, 0.75, 1, 1)
+
+[node name="EditorLabel" type="Label" parent="UI"]
+mouse_filter = 2
+anchor_left = 0.5
+anchor_right = 0.5
+anchor_top = 0.5
+anchor_bottom = 0.5
+offset_left = -200
+offset_right = 200
+offset_top = 120
+offset_bottom = 186
+theme_override_font_sizes/font_size = 32
+horizontal_alignment = 1
+text = "Map Editor"
+
+[node name="StatusLabel" type="Label" parent="UI"]
+anchor_left = 0.5
+anchor_right = 0.5
+anchor_top = 0.5
+anchor_bottom = 0.5
+offset_left = -260
+offset_right = 260
+offset_top = 200
+offset_bottom = 260
+theme_override_font_sizes/font_size = 26
+horizontal_alignment = 1
+autowrap_mode = 2
+text = ""
+
+[node name="QuitButton" type="Control" parent="UI"]
+anchor_left = 0.5
+anchor_right = 0.5
+anchor_top = 0.5
+anchor_bottom = 0.5
+offset_left = -64
+offset_right = 64
+offset_top = 320
+offset_bottom = 448
+script = ExtResource("2")
+icon = 1
+glow_color = Color(0.9, 0.35, 0.35, 1)
+
+[node name="QuitLabel" type="Label" parent="UI"]
+mouse_filter = 2
+anchor_left = 0.5
+anchor_right = 0.5
+anchor_top = 0.5
+anchor_bottom = 0.5
+offset_left = -160
+offset_right = 160
+offset_top = 460
+offset_bottom = 526
+theme_override_font_sizes/font_size = 32
+horizontal_alignment = 1
+text = "Quit"