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>

CREDITS.md

@@ -1,8 +1,14 @@
 # Credits
 
 Every third-party asset pack used or bundled during development, credited
-regardless of whether its licence requires it. Two of them do not — they are
-here because the work deserves it.
+regardless of whether its licence requires it. Some of them do not require it —
+they are here because the work deserves it.
+
+**This file is not the only place credit appears.** Two of the packs are CC BY
+4.0, which asks for attribution "in any reasonable manner", and a markdown file
+in a source repository is not reasonable for someone who downloaded a build. The
+game carries the same list on its own credits screen (Settings → Credits), built
+from `src/core/credits.gd`; `tests/unit/test_credits.gd` asserts the two agree.
 
 ## In the game
 
@@ -11,6 +17,12 @@ Terrain, player, enemies and boss sprites.
 [CC0 1.0](https://creativecommons.org/publicdomain/zero/1.0/) — public domain.
 *Credit not required.*
 
+**Complete UI Essential Pack** — [Crusenho Agus Hennihuno](https://crusenho.itch.io/complete-ui-essential-pack)
+Buttons, panels, bars and inventory slots. Used unmodified; a subset of the
+pack's individual sprites is committed under `assets/sprites/ui/`.
+[CC BY 4.0](https://creativecommons.org/licenses/by/4.0/).
+**Credit required by licence.**
+
 **Pixel Combat SFX** — [Helton Yan](https://heltonyan.itch.io/pixelcombat)
 All sound effects. Converted to 16-bit / 44.1 kHz mono and trimmed.
 [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/).