docs: switch asset pipeline notes to AssetServer model

Card faces, card backs, board backgrounds, and the UI font are loaded
via Bevy's AssetServer at startup (see commit fbe984c). The CLAUDE.md
hard rule still claimed cards/backgrounds were rendered procedurally
with no AssetServer, and ARCHITECTURE.md §14 / §20 still described
PNGs and TTFs as embedded via include_bytes!(). Update both docs:

- CLAUDE.md hard rule lists which assets ship in assets/ and notes the
  Option<Res<AssetServer>> fallback used under MinimalPlugins (tests).
- ARCHITECTURE.md §2/§3/§5/§14 rewritten to describe the AssetServer
  loaders for CardImageSet, BackgroundImageSet, and FontResource, and
  the Text2d / solid-colour fallbacks.
- ARCHITECTURE.md §20 decision log replaces the two reversed
  embed-via-include_bytes!() entries with a single entry covering the
  switch to AssetServer plus a note that audio remains embedded.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -47,7 +47,9 @@ cargo clippy -p solitaire_core -- -D warnings
 
 - `solitaire_core` and `solitaire_sync` must never gain Bevy or network dependencies.
 - No `unwrap()` or `panic!()` in game logic. All state transitions return `Result<_, MoveError>`.
-- Audio assets are embedded at compile time using `include_bytes!()` in `audio_plugin.rs`. Cards and backgrounds are rendered procedurally (colored `Sprite` entities + text) — no image files are used and no `AssetServer` is needed.
+- Audio assets are embedded at compile time using `include_bytes!()` in `audio_plugin.rs`.
+- Card faces (52 PNGs in `assets/cards/faces/`), card backs (`assets/cards/backs/back_N.png`), board backgrounds (`assets/backgrounds/bg_N.png`), and the UI font (`assets/fonts/main.ttf`) are loaded at runtime via `AssetServer::load()` and stored as `Handle<Image>`/`Handle<Font>` in the `CardImageSet`, `BackgroundImageSet`, and `FontResource` resources. The `assets/` directory must ship alongside the binary.
+- Asset-loading systems take `Option<Res<AssetServer>>` so they degrade cleanly under `MinimalPlugins` (tests). When `CardImageSet` is absent, `card_plugin` falls back to a `Text2d` rank+suit overlay; when `BackgroundImageSet` is absent, the board falls back to a solid colour.
 - Atomic file writes only: write to `filename.json.tmp`, then `rename()`.
 - Passwords and tokens are stored in the OS keychain via the `keyring` crate — never in plaintext files or logs.
 - Sync runs on `AsyncComputeTaskPool` — never block the Bevy main thread.