rnd(M03): add rendered ground query facade

src/render/terrain/rendered_ground_sample.gd

@@ -0,0 +1,50 @@
+class_name RenderedGroundSample
+extends RefCounted
+
+## Immutable result of sampling currently loaded render terrain in Godot world units.
+## This diagnostic value is renderer-owned and is not authoritative gameplay collision.
+
+var is_available: bool:
+	get:
+		return _is_available
+
+var height_units: float:
+	get:
+		return _height_units
+
+var failure_code: StringName:
+	get:
+		return _failure_code
+
+var _is_available: bool
+var _height_units: float
+var _failure_code: StringName
+
+
+## Creates an available finite rendered-ground sample in Godot world units.
+static func available(height_units_value: float) -> RenderedGroundSample:
+	return RenderedGroundSample.new(true, height_units_value, &"")
+
+
+## Creates an unavailable rendered-ground sample with a stable failure code.
+static func unavailable(failure_code_value: StringName) -> RenderedGroundSample:
+	return RenderedGroundSample.new(false, 0.0, failure_code_value)
+
+
+func _init(
+		is_available_value: bool,
+		height_units_value: float,
+		failure_code_value: StringName
+) -> void:
+	_is_available = is_available_value and is_finite(height_units_value)
+	_height_units = height_units_value if _is_available else 0.0
+	if _is_available:
+		_failure_code = &""
+	elif not is_finite(height_units_value):
+		_failure_code = &"render_terrain_height_not_finite"
+	else:
+		_failure_code = (
+			failure_code_value
+			if not failure_code_value.is_empty()
+			else &"render_terrain_unavailable"
+		)