Give the gas planet surface its own background shader (#51)

lunar_surface.gdshader recoloured was the whole gas surface backdrop,
so a landed gas giant read as regolith - the crater field never went
away, just changed colour. New gas_surface.gdshader replaces it for
gas profiles only: a nearest-row-wins horizontal band field (same
single-scalar discipline as lunar's nearest-crater field), with
drifting per-row turbulence so the bands ripple instead of reading as
ruled lines. Keeps the exact same uniform interface as lunar
(camera_cell/camera_frac/camera_zoom/viewport_size/cell_size/
base_color/crater_color/highlight_color), so LunarBackground.gd needs
no changes at all.

PlanetSurfaceProfile gains background_shader (default null = keep the
scene's existing lunar shader); PlanetSurfaceController._compose_surface()
swaps the background material's shader in only when a profile names one.
GasSurfaceProfile.tres points at the new shader and retunes its palette
to a deep-atmosphere/shadowed-band/lit-crest read; RockySurfaceProfile.tres
is untouched, so the rocky surface renders exactly as before.

Precision: every hash input is either an exact integer cell/row index or
a bounded [0,1) local fraction, same split as lunar_surface.gdshader and
Starfield.gd use for the same reason (world coordinates reach the tens of
thousands, where mobile float precision collapses). TIME-based lateral
drift is folded into the lattice coordinate via the same floor/remainder
trick rather than ever being added to a large cell index directly - see
combine_lattice()'s header note.

Tests: new tests/test_surface_profiles.gd (rocky resolves no
background_shader and keeps the lunar shader; gas resolves and applies
its own, different from lunar's). Full suite: 78/78 passed. Screenshot
of the gas surface confirms drifting bands with no craters, rendered
well inside the softpipe performance budget (20 frames in ~2m40s,
against lunar's "minutes per frame" baseline the ticket warns about).

Repo notes for the next agent, confirmed empirically this session:
- Never reference a plain (non-autoload) class_name class by its bare
  global name from a -s entry-point test script - `load()` it by path
  into a local var instead, same as PlanetSurfaceController.gd and
  PlanetSetups.gd already do for exactly this reason. Doing it the
  unsafe way here didn't fail to compile - it corrupted something in
  the resource-loading state badly enough that an UNRELATED scene's
  Rock.tscn lost its script attachment and RockField's rock-spawning
  loop exhausted the physics server's entire RID pool. If a test's
  crash trace implicates code you never touched, check every bare
  global class_name reference in the test first.
- Running a `tests/screenshot.sh` render (Mesa softpipe, genuinely
  minutes for a crater-shader scene) at the same time as a
  `run_tests.sh` full-suite run starves both for CPU on this device -
  kill one before waiting on the other rather than running them
  concurrently.

🤖 Generated with Claude Code

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

tests/_rocky_surface_preview.gd

@@ -0,0 +1,20 @@
+extends Node2D
+
+# Preview, not a test - #51's "the rocky surface is unchanged" acceptance
+# criterion. Mirrors _gas_surface_preview.gd exactly except for the planet
+# type, so the two screenshots are a fair side-by-side comparison.
+#
+#   bash tests/screenshot.sh res://tests/_rocky_surface_preview.tscn /tmp/rocky.png 20 720x1280
+
+
+func _ready() -> void:
+	GameState.planet_name = "Kepler's Rest"
+	GameState.planet_definition_id = "rocky"
+	GameState.surviving_ship_count = 3
+	GameState.rocks = 128
+	GameState.gas = 46
+	GameState.fleet_cargo = 34
+	GameState.fleet_gas_cargo = 17
+	var surface: Node = load("res://PlanetSurface.tscn").instantiate()
+	surface.get_node("SurfaceController").fade_duration = 0.0
+	add_child(surface)