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/hud_plugin.rs

@@ -14,6 +14,7 @@ use solitaire_core::pile::PileType;
 use crate::auto_complete_plugin::AutoCompleteState;
 use crate::daily_challenge_plugin::DailyChallengeResource;
 use crate::events::InfoToastEvent;
+use crate::font_plugin::FontResource;
 use crate::game_plugin::GameMutation;
 use crate::resources::GameStateResource;
 use crate::selection_plugin::SelectionState;
@@ -98,9 +99,13 @@ impl Plugin for HudPlugin {
     }
 }
 
-fn spawn_hud(mut commands: Commands) {
+fn spawn_hud(font_res: Option<Res<FontResource>>, mut commands: Commands) {
     let white = TextColor(Color::srgba(1.0, 1.0, 1.0, 0.80));
-    let font = TextFont { font_size: 18.0, ..default() };
+    let font = TextFont {
+        font: font_res.as_ref().map(|f| f.0.clone()).unwrap_or_default(),
+        font_size: 18.0,
+        ..default()
+    };
     commands
         .spawn((
             Node {