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>

docs/WORKFLOW.md

@@ -251,3 +251,24 @@ the output, which is worth doing after any change to startup.
 `smoke.sh` now asserts no plain engine errors either, excluding by name the one
 line Godot prints on every clean exit — naming it means anything else that turns
 up is a real finding rather than noise to be squinted past.
+
+### Nothing automated can see the screen
+
+Every gate passed — 458 tests, a clean smoke run, no engine errors — while the
+entire interface rendered in Godot's default style and none of the UI pack was
+visible. A Control inherits its theme from Control *ancestors only*, and the
+chain breaks at the first plain `Node` or `CanvasLayer`; every screen in this
+game hangs off one. `get_window().theme = ...` set the property, changed
+nothing, and looked correct in the code.
+
+It took `tools/screenshot.tscn` — boot the client, save a PNG, open it — to
+find. The same run then showed **Fire bound to the right mouse button**, which
+was the test suite writing the player's real `user://settings.cfg`, because
+rebinding calls `save()` and nothing had redirected the path.
+
+Two lessons, and the second is the general one:
+
+- After touching `src/ui/`, look at it. There is no substitute.
+- A test that exercises a code path which writes to user state **will write to
+  user state**. Redirect the path in the fixture, and assert the default is
+  still the real one so the redirect cannot escape.