gmp(M02): extract player terrain query

Work-Package: M02-GMP-TERRAIN-QUERY-001
Agent: sindo-main-codex
Tests: terrain query, player input and movement regressions; asset-free scene; renderer dry-run; coordinate, streaming, documentation and coordination gates
Fidelity: preserves existing ADT outer-grid interpolation and player ground snap; build-12340 collision parity remains unverified

src/gameplay/terrain/terrain_ground_sample.gd

@@ -0,0 +1,49 @@
+class_name TerrainGroundSample
+extends RefCounted
+
+## Immutable result of a terrain ground-height query in Godot world units.
+## An unavailable sample carries a stable failure code instead of overloading a
+## numeric height with `NAN` at the public boundary.
+
+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 ground sample in Godot world units.
+static func available(height_units_value: float) -> TerrainGroundSample:
+	return TerrainGroundSample.new(true, height_units_value, &"")
+
+
+## Creates an unavailable sample with a stable diagnostic code.
+static func unavailable(failure_code_value: StringName) -> TerrainGroundSample:
+	return TerrainGroundSample.new(false, 0.0, failure_code_value)
+
+
+## Normalizes factory inputs so available samples are finite and unavailable
+## samples always carry a non-empty failure code.
+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 = &"terrain_height_not_finite"
+	else:
+		_failure_code = failure_code_value if not failure_code_value.is_empty() else &"terrain_unavailable"