Make the lunar backdrop cheap: 3x3 field, then bake and tile it (#68)

The crater shader ran on every pixel of a fullscreen rect every frame and
was by far the most expensive thing in the surface scene. Nothing else came
close: RockField keeps 14 rocks alive, Rock/Ship/Enemy already avoid
per-frame queue_redraw() for transform-only changes, and LunarBackground.gd
is a handful of scalar ops.

Three reductions to the field itself:

- 5x5 -> 3x3 neighbour scan. That width was never the geometric answer;
  it was insurance against craters seen clipping at cell boundaries
  on-device, i.e. a crater reaching past its nominal radius because a hash
  returned a value outside [0,1] and extrapolated mix(). Clamping the
  crater CENTRE into its own cell, alongside the radius clamp already
  there, bounds the reach whatever the hash returns - so a crater two cells
  out is >= 1.0 cells away, a normalized distance of >= 2.63, past every
  threshold the shading reacts to (the last saturates at 1.15). The outer
  ring provably cannot change a pixel. This removes the clipping bug's
  cause instead of hiding it behind 16 extra samples.
- One hash42 per cell instead of hash21 twice plus hash22, decorrelating
  on output lanes rather than input offsets.
- Squared distances in the loop, square-rooted twice at the end rather
  than once per sample.

Measured with the new tests/profile_shader.sh, above softpipe's own ~657ms
fullscreen floor: 9482ms -> 405ms of shader work, a 23x reduction.

Then the larger change: the field is a fixed function of world position, so
every frame recomputed an identical answer. It is now made periodic (the
cell index is wrapped before hashing, which makes a Worley field tile
exactly rather than merely blend), baked once at load into one tile, and
scrolled with repeat wrapping by lunar_tiled.gdshader - one texture fetch
per pixel. Two honest costs: the field repeats every 7040 world units, and
a fixed-resolution tile softens when zoomed past 1:1. Lunar regolith is
forgiving of both, having no landmarks and being smooth gradients already.

The bake waits a frame and asks the material whether it actually samples a
field_texture, so a gas giant - which swaps in its own background shader
(#51) - does not pay for a 2048-square tile it will never read. Headless
runs skip it entirely, having nothing to rasterize.

tests/test_lunar_field.gd guards the reasoning, since a fragment shader
cannot be asserted on directly: the premises are still in the source, the
worst-case ring-2 geometry stays clear of every threshold independent of
the hash, a 3x3 and a 5x5 scan render identically over 4000 sampled points,
the field repeats exactly in both axes, and the miss sentinel still cannot
overflow mediump into +Inf and then NaN.

test_surface_profiles.gd asserted "a rocky landing leaves the background
alone" by naming lunar_surface.gdshader; it now reads the shader from the
scene, which is the property it actually cares about.

PlanetSurface.tscn's edit also carries the planet-name relocation described
in the HUD commit - same scene file, so the two cannot be split.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/test_surface_profiles.gd

@@ -19,7 +19,7 @@ extends SceneTree
 # rather than checking on the same one.
 
 var game_state: Node
-var lunar_shader: Shader
+var scene_shader: Shader
 var gas_shader: Shader
 var rocky_shader_after_compose: Shader
 var scene: Node
@@ -29,7 +29,17 @@ var elapsed: float = 0.0
 
 func _initialize() -> void:
 	game_state = root.get_node("GameState")
-	lunar_shader = load("res://lunar_surface.gdshader")
+	# Whatever PlanetSurface.tscn itself ships with, read from the scene
+	# rather than named by path. The property under test is "a null
+	# background_shader leaves the scene's own background alone" - naming a
+	# specific file here instead made this fail the moment the lunar
+	# background changed implementation (#68 swapped the per-pixel
+	# procedural shader for a baked, tiled one), which is a change this test
+	# has no business having an opinion about.
+	var packed: PackedScene = load("res://PlanetSurface.tscn")
+	var probe: Node = packed.instantiate()
+	scene_shader = probe.get_node("BackgroundLayer/LunarBackground").material.shader
+	probe.free()
 
 	# Loaded by path into a local, not referenced by its bare global
 	# class_name - same load-order hazard PlanetSetups.gd and
@@ -47,8 +57,8 @@ func _initialize() -> void:
 		quit(1)
 		return
 	gas_shader = gas_profile.background_shader
-	if gas_shader == lunar_shader:
-		print("TEST_SURFACE_PROFILES: FAIL (gas profile points at the lunar shader, not its own)")
+	if gas_shader == scene_shader:
+		print("TEST_SURFACE_PROFILES: FAIL (gas profile points at the scene's own shader, not its own)")
 		quit(1)
 		return
 
@@ -79,7 +89,7 @@ func _process(delta: float) -> bool:
 			var gas_bg: ShaderMaterial = scene.get_node("BackgroundLayer/LunarBackground").material
 			scene.free()
 
-			if rocky_shader_after_compose != lunar_shader:
+			if rocky_shader_after_compose != scene_shader:
 				print("TEST_SURFACE_PROFILES: FAIL (landing on a rocky planet changed the background shader)")
 				quit(1)
 				return true
@@ -87,8 +97,8 @@ func _process(delta: float) -> bool:
 				print("TEST_SURFACE_PROFILES: FAIL (landing on a gas giant did not swap in gas_surface.gdshader)")
 				quit(1)
 				return true
-			if gas_bg.shader == lunar_shader:
-				print("TEST_SURFACE_PROFILES: FAIL (gas surface still uses the lunar shader)")
+			if gas_bg.shader == scene_shader:
+				print("TEST_SURFACE_PROFILES: FAIL (gas surface still uses the scene's own lunar background)")
 				quit(1)
 				return true