feat(sonar): add SAND terrain material

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

tests/unit/run_sonar_application_tests.gd

@@ -20,7 +20,7 @@ const SonarAcquisitionPort = preload(
 )
 const SonarService = preload("res://game/application/sonar/sonar_service.gd")
 
-const TEST_COUNT := 32
+const TEST_COUNT := 34
 
 var _failures: Array[String] = []
 
@@ -76,6 +76,16 @@ func _test_sample_buffer_contract() -> void:
 		true
 	)
 	_expect(sample.validation_error().is_empty(), "observed typed sample is valid")
+	var sand_sample := SonarSample.new(
+		Vector3(2.0, -4.0, 9.0),
+		Vector3.UP,
+		0.4,
+		-4.0,
+		SonarMaterial.Value.SAND,
+		true
+	)
+	_expect(sand_sample.validation_error().is_empty(), "sand is a valid terrain material")
+	_expect(SonarMaterial.is_valid(SonarMaterial.Value.SAND), "sand is inside the material range")
 	var buffer := SonarSampleBuffer.new(3, 2)
 	_expect(buffer.set_sample(1, 2, sample).is_empty(), "sample writes inside buffer bounds")
 	_expect(buffer.observed_count() == 1, "buffer counts observed cells")