feat(engine): rename themes — Classic is default, Dark replaces Default

- Rename assets/themes/default/ → assets/themes/dark/; update theme.ron
  id/name to "dark"/"Dark"
- Rename all DEFAULT_THEME_* constants → DARK_THEME_* and
  default_theme_svg_bytes / populate_embedded_default_theme → dark_*
- Add bundled_theme_url() helper for URL resolution without needing the
  registry (used by Startup systems where ordering isn't guaranteed)
- Registry now lists Classic first (new player default), Dark second
- settings.rs default_theme_id() returns "classic" so fresh installs
  start on the white card theme

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_engine/assets/themes/dark/PROVENANCE.md

@@ -0,0 +1,43 @@
+# Default theme — provenance
+
+This directory is the bundled-default card theme that ships embedded in
+the binary via Bevy's `embedded_asset!` macro (see
+`solitaire_engine/src/assets/sources.rs`). At runtime its files are
+addressable as `embedded://solitaire_engine/assets/themes/default/...`.
+
+## Current state (Phase 3)
+
+The `theme.ron` manifest in this directory lists all 52 face slots plus
+a back slot, but **the referenced SVG files do not yet exist**. The
+manifest is intentionally a stub so that:
+
+1. `embedded_asset!` has a real file to bundle (the manifest itself).
+2. `ThemeManifest::validate` accepts the manifest (it requires all 52
+   faces to be listed by name).
+3. The `embedded://` asset source can be source-registered and queried
+   without runtime errors during Phase 3.
+
+The actual SVG art will be added when the project swaps in the
+`hayeah/playing-cards-assets` artwork — see the implementation plan in
+`/CARD_PLAN.md`. At that point, every `.svg` filename listed in
+`theme.ron`'s `faces` map (and `back.svg`) must be added here, and each
+new file needs a corresponding `embedded_asset!(app, ...)` call in
+`solitaire_engine/src/assets/sources.rs::register_default_theme`.
+
+## How to add files to the bundled default theme
+
+For each new file you drop into this directory:
+
+1. Drop the file under `solitaire_engine/assets/themes/default/`.
+2. Add one line to `register_default_theme` in
+   `solitaire_engine/src/assets/sources.rs` of the form:
+   ```rust
+   embedded_asset!(app, "../../assets/themes/default/<filename>");
+   ```
+   (The path is relative to `sources.rs`, which lives in
+   `solitaire_engine/src/assets/`.)
+3. Update this file with the licence and origin of the new asset.
+
+## Licence
+
+To be filled in once real artwork lands.