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_query.gd

@@ -0,0 +1,12 @@
+class_name TerrainQuery
+extends RefCounted
+
+## Replaceable scene-free boundary for gameplay terrain height sampling.
+## Implementations receive a typed Godot world position and must return an
+## explicit available/unavailable result without mutating the caller.
+
+
+## Samples terrain ground height at a typed Godot world position.
+## The base implementation reports `terrain_query_not_implemented`.
+func sample_ground_height(_godot_world_position: GodotWorldPosition) -> TerrainGroundSample:
+	return TerrainGroundSample.unavailable(&"terrain_query_not_implemented")