feat(ambience): underwater shaders + audio manager + main menu + CC0 assets

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/ambience/UnderwaterEnvironment.gd

@@ -0,0 +1,44 @@
+extends WorldEnvironment
+
+func _ready() -> void:
+	var env := Environment.new()
+
+	env.background_mode = Environment.BG_COLOR
+	env.background_color = Color(0.03, 0.12, 0.22)
+
+	env.ambient_light_source = Environment.AMBIENT_SOURCE_COLOR
+	env.ambient_light_color = Color(0.2, 0.4, 0.6)
+	env.ambient_light_energy = 0.4
+
+	env.fog_enabled = true
+	env.fog_light_color = Color(0.1, 0.3, 0.5)
+	env.fog_density = 0.02
+	env.fog_aerial_perspective = 0.3
+
+	env.volumetric_fog_enabled = true
+	env.volumetric_fog_density = 0.04
+	env.volumetric_fog_albedo = Color(0.2, 0.5, 0.7)
+	env.volumetric_fog_emission = Color(0.02, 0.06, 0.1)
+	env.volumetric_fog_emission_energy = 0.1
+	env.volumetric_fog_length = 64.0
+	env.volumetric_fog_detail_spread = 2.0
+
+	env.glow_enabled = true
+	env.glow_intensity = 0.5
+	env.glow_bloom = 0.1
+	env.glow_blend_mode = Environment.GLOW_BLEND_MODE_SOFTLIGHT
+
+	env.ssao_enabled = true
+	env.ssao_radius = 1.0
+	env.ssao_intensity = 1.0
+	env.ssao_power = 1.5
+	env.ssao_detail = 0.5
+
+	env.tone_mapper = Environment.TONE_MAPPER_FILMIC
+	env.exposure = 1.0
+
+	env.adjustment_enabled = true
+	env.adjustment_saturation = 1.1
+	env.adjustment_color_correction = null
+
+	environment = env