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/ui/settings_screen.gd

@@ -6,6 +6,7 @@ extends CanvasLayer
 ## game's state that should change what it can do.
 
 signal closed
+signal credits_requested
 
 var _rows: Dictionary[String, Button] = {}
 var _status: Label
@@ -20,8 +21,7 @@ var _listening: String = ""
 func _ready() -> void:
 	layer = 32
 	visible = false
-	var root := Control.new()
-	root.set_anchors_and_offsets_preset(Control.PRESET_FULL_RECT)
+	var root := UiTheme.themed_root()
 	add_child(root)
 
 	var scrim := ColorRect.new()
@@ -81,6 +81,11 @@ func _ready() -> void:
 		_status.text = "controls reset"
 		refresh())
 	buttons.add_child(reset)
+	var credits := Button.new()
+	credits.text = "Credits"
+	credits.custom_minimum_size = Vector2(160.0, 34.0)
+	credits.pressed.connect(func() -> void: credits_requested.emit())
+	buttons.add_child(credits)
 	var close := Button.new()
 	close.text = "Close"
 	close.custom_minimum_size = Vector2(160.0, 34.0)