feat(engine): art pass — PNG assets, custom font, and keyring v4 upgrade

Art pass (Phase 4):
- Generate placeholder PNG assets: face.png, back_0–4.png, bg_0–4.png via
  solitaire_assetgen gen_art binary (16×16 RGBA, embedded via include_bytes!)
- Add FiraMono-Medium font (assets/fonts/main.ttf) embedded at compile time
- Add FontPlugin: loads font at startup, exposes FontResource; gracefully
  falls back to default handle when Assets<Font> absent (MinimalPlugins tests)
- Wire CardImageSet into card_plugin: face/back PNGs replace solid-colour
  sprites when available; tests continue using colour fallback via MinimalPlugins
- Wire BackgroundImageSet into table_plugin: bg PNGs replace solid-colour
  background; empty set inserted when Assets<Image> absent in tests
- Fix hint highlight system (input_plugin): tint sprite.color directly instead
  of replacing the whole Sprite (which would discard the image handle)
- Export FontPlugin, FontResource, CardImageSet from solitaire_engine::lib
- Register FontPlugin in solitaire_app before other plugins

Dependency upgrades (latest releases):
- keyring "2" → keyring "4" + keyring-core "1" (v4 split architecture into
  separate core library crate)
- auth_tokens.rs: Entry::new now returns Result; delete_password →
  delete_credential; NoDefaultStore error variant handled
- solitaire_app: add keyring::use_native_store(true) at startup for Linux
  Secret Service / macOS Keychain / Windows Credential Store selection

ARCHITECTURE.md: fix Edition 2025→2021, update asset pipeline section,
add FontPlugin/CardImageSet/BackgroundImageSet to plugin and resource tables,
update Section 14 to reflect actual include_bytes!() rendering approach,
add Decision Log entries for embedded PNG and font decisions

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

solitaire_engine/src/lib.rs

@@ -6,6 +6,7 @@ pub mod animation_plugin;
 pub mod auto_complete_plugin;
 pub mod audio_plugin;
 pub mod card_plugin;
+pub mod font_plugin;
 pub mod feedback_anim_plugin;
 pub mod challenge_plugin;
 pub mod cursor_plugin;
@@ -59,9 +60,10 @@ pub use feedback_anim_plugin::{
 pub use auto_complete_plugin::AutoCompletePlugin;
 pub use audio_plugin::{AudioPlugin, AudioState, SoundLibrary};
 pub use card_plugin::{
-    CardEntity, CardLabel, CardPlugin, HintHighlight, HintHighlightTimer, RightClickHighlight,
-    RightClickHighlightTimer,
+    CardEntity, CardImageSet, CardLabel, CardPlugin, HintHighlight, HintHighlightTimer,
+    RightClickHighlight, RightClickHighlightTimer,
 };
+pub use font_plugin::{FontPlugin, FontResource};
 pub use cursor_plugin::CursorPlugin;
 pub use events::{
     AchievementUnlockedEvent, CardFaceRevealedEvent, CardFlippedEvent, DrawRequestEvent,