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>

CLAUDE.md

@@ -29,6 +29,7 @@ godot --headless --path . res://tools/diag_loot.tscn          # drop -> pick up
 godot --headless --path . res://tools/diag_upgrades.tscn      # level -> choice -> taken at the NPC -> new stats
 godot --headless --path . --script tools/bench.gd             # sim cost per tick
 python3 tools/build_local_assets.py                           # rebuild the local-only bullet atlas
+SHOT_DIR=/tmp/shots godot --path . res://tools/screenshot.tscn # capture the menus and HUD (needs a display)
 ```
 
 `diag_progression`, `diag_loot` and `diag_upgrades` exist because the bot smoke
@@ -105,6 +106,8 @@ and `tests/integration/test_replica_parity.gd` pin this down.
 | `src/instances/` | Lobby hub and dungeon runs. |
 | `src/view/`, `src/ui/` | Read-only rendering. Never decides anything. |
 | `src/core/settings.gd` | Client-local preferences: key bindings and volumes. Never reaches the server. |
+| `src/core/credits.gd` | Every third-party asset and its licence. Two are CC BY, so this is a legal requirement, not a nicety. |
+| `src/view/ui_theme.gd` | The control theme, built in code from the UI pack. |
 | `src/autoload/net.gd` | The only autoload. RPC surface. |
 | `tools/` | Headless tooling. |
 
@@ -137,7 +140,15 @@ ticks in milliseconds with no SceneTree.
    coordinate even on an anchored control; `offset_left`/`offset_top` are the
    anchor-relative ones. This combination silently broke the main menu and
    three pieces of the HUD.
-6. **`ProjectSettings.save()` drops settings equal to the engine default** and
+6. **A Control's theme is inherited from Control ANCESTORS only.** The chain
+   breaks at the first parent that is a plain `Node` or a `CanvasLayer`, which
+   here is every screen: they hang off `main.gd` or off a CanvasLayer. Setting
+   `get_window().theme` therefore compiles, runs, changes the property — and
+   styles nothing. Apply the theme to each screen's own root Control
+   (`UiTheme.themed_root()`). `tests/unit/test_ui_theme.gd` instantiates every
+   screen and asks what its buttons actually resolve, because this failure is
+   invisible to every other kind of check.
+7. **`ProjectSettings.save()` drops settings equal to the engine default** and
    strips comments. Anything load-bearing (the 60 Hz tick) is asserted in code
    in `src/main.gd` instead of trusted to `project.godot`.