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/font_plugin.rs

@@ -0,0 +1,36 @@
+// Register FontPlugin in solitaire_engine/src/lib.rs before use.
+
+//! Embeds FiraMono-Medium as the project font and exposes it via [`FontResource`].
+
+use bevy::prelude::*;
+
+/// Holds the project-wide [`Handle<Font>`] loaded at startup.
+#[derive(Resource)]
+pub struct FontResource(pub Handle<Font>);
+
+/// Loads FiraMono-Medium at startup and inserts [`FontResource`].
+pub struct FontPlugin;
+
+impl Plugin for FontPlugin {
+    fn build(&self, app: &mut App) {
+        app.add_systems(Startup, load_font);
+    }
+}
+
+fn load_font(fonts: Option<ResMut<Assets<Font>>>, mut commands: Commands) {
+    let Some(mut fonts) = fonts else {
+        // Assets<Font> absent (e.g. MinimalPlugins in tests) — insert default.
+        commands.insert_resource(FontResource(Handle::default()));
+        return;
+    };
+    let bytes: &'static [u8] = include_bytes!("../../assets/fonts/main.ttf");
+    match Font::try_from_bytes(bytes.to_vec()) {
+        Ok(font) => {
+            commands.insert_resource(FontResource(fonts.add(font)));
+        }
+        Err(e) => {
+            warn!("failed to load main.ttf: {e}; falling back to Bevy default font");
+            commands.insert_resource(FontResource(Handle::default()));
+        }
+    }
+}