Rework the UI on Crusenho's pack; credit it in the game, not just the repo

Crusenho's Complete UI Essential Pack is CC BY 4.0 -- redistributable and
commercial-friendly, confirmed from the License.txt the pack itself ships --
so unlike the two bdragon packs a subset is committed: twelve PNGs, 48 KB,
under assets/sprites/ui/. Only what is used, because each committed PNG costs
a Godot .import sidecar and a directory nothing references is one nobody
prunes.

UiTheme builds a Theme in code from it -- button states, panels, line edits --
and every screen roots itself through UiTheme.themed_root(). The HUD's bars are
the pack's frame with a tinted fill, drawn as three horizontal slices because
Godot's nine-patch lives on nodes and the HUD is drawn rather than built from
controls. Inventory slots use the pack's slot art at exactly twice the source
size; a non-integer scale on a 1px border reads as a wobble along every edge.

The credits screen is the other half of the request and it is a licence
obligation, not a nicety: two packs are now CC BY, which asks for attribution
"in any reasonable manner", and a markdown file in a source repo is not
reasonable for someone who downloaded a build. Settings -> Credits shows every
source with its terms and a link to the licence text. test_credits.gd asserts
CREDITS.md and docs/ASSETS.md name every entry, so the three cannot drift.

Two things found by actually looking at the screen, which is the point:

  - The FIRST version of this styled nothing. A Control inherits its theme from
    Control ANCESTORS only, and the chain breaks at the first plain Node or
    CanvasLayer -- which is every screen here. get_window().theme set the
    property, changed nothing, and read as correct. check.sh, 458 tests and a
    clean smoke run all passed with the entire interface unstyled. The theme
    test now instantiates every screen and asks what its buttons resolve.
  - The settings screen showed Fire bound to the right mouse button, because
    the test suite was writing the player's real user://settings.cfg --
    rebinding calls save() and nothing had redirected the path. Settings.path
    is now redirectable, the fixture points it at a scratch file, and a test
    asserts the default is still the player's own.

tools/screenshot.tscn is what found both. It boots the client windowed and
saves the menus, the HUD, settings and credits. Manual, needs a display, and
the only thing in the project that can tell you the interface rendered.

check.sh clean, 460 tests, SMOKE PASS, all four diagnostics green.

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

src/core/settings.gd

@@ -10,7 +10,14 @@ extends RefCounted
 ## tool has no main loop and cannot resolve autoload names, and the settings
 ## file is something a headless test may well want to read.
 
-const PATH := "user://settings.cfg"
+const DEFAULT_PATH := "user://settings.cfg"
+
+## Where preferences are read from and written to. A variable rather than a
+## constant so tests can point it at a scratch file: rebinding calls save(),
+## and a suite run was otherwise rewriting the player's real settings -- which
+## it did, silently, until a screenshot showed Fire bound to the right mouse
+## button.
+static var path: String = DEFAULT_PATH
 
 ## Actions the settings screen offers, in the order it lists them. Anything not
 ## here keeps whatever `tools/setup_input_map.gd` gave it and cannot be changed
@@ -54,7 +61,7 @@ static var _defaults_captured: bool = false
 static func load_and_apply() -> void:
 	_capture_defaults()
 	var cfg := ConfigFile.new()
-	if cfg.load(PATH) == OK:
+	if cfg.load(path) == OK:
 		master_volume = clampf(float(cfg.get_value("audio", "master", master_volume)), 0.0, 1.0)
 		sfx_volume = clampf(float(cfg.get_value("audio", "sfx", sfx_volume)), 0.0, 1.0)
 		bindings.clear()
@@ -81,9 +88,9 @@ static func save() -> void:
 	cfg.set_value("audio", "sfx", sfx_volume)
 	for action in bindings:
 		cfg.set_value("input", action, bindings[action])
-	var err := cfg.save(PATH)
+	var err := cfg.save(path)
 	if err != OK:
-		GameLog.warn("settings", "could not write %s (error %d)" % [PATH, err])
+		GameLog.warn("settings", "could not write %s (error %d)" % [path, err])
 
 
 # --- Audio -------------------------------------------------------------------