feat(engine): per-theme nearest-sampling opt-in for pixel-art themes

Bevy's default sprite sampler is bilinear (Linear), which mushes
pixel-art card faces at non-integer scales. The rusty-pixel theme
ships 256x384 source PNGs that get displayed at ~150-200px wide on
typical desktop windows — an aggressive downscale where bilinear
visibly blurs the pixel grid.

Globally flipping ImagePlugin to default_nearest() would also affect
the SVG-rasterised default theme, where bilinear's smoothing is
actually desired (the SVG rasteriser produces a high-res 512x768
pixmap that the GPU has to downscale at draw time).

The fix is a per-theme opt-in:

  - ThemeMeta gains pixel_art: bool with #[serde(default)] for
    backwards compat. Older manifests load with `false`, preserving
    SVG-default behaviour.
  - sync_card_image_set_with_active_theme inspects theme.meta.pixel_art
    after a theme finishes loading. When true, walks every face +
    back Handle<Image> in the active CardTheme and rewrites its
    sampler to ImageSampler::Descriptor(ImageSamplerDescriptor::nearest()).
    The Modified asset event triggers a GPU re-upload with the new
    sampler descriptor.
  - The 12 ThemeMeta struct literals across the engine
    (settings_plugin, card_plugin, theme/{plugin,mod,manifest,
    importer,registry}) all gain `pixel_art: false` to match the
    new field.

The deployed rusty-pixel theme.ron at
~/.local/share/solitaire_quest/themes/rusty-pixel/ now sets
pixel_art: true, so the player's switch-to-pixel-art chip flips to
nearest sampling on the spot.

Workspace: 1171 passing tests / 0 failing. cargo clippy
--workspace --all-targets -- -D warnings clean.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

solitaire_engine/src/theme/plugin.rs

@@ -187,6 +187,7 @@ fn sync_card_image_set_with_active_theme(
     themes: Res<Assets<CardTheme>>,
     mut card_image_set: Option<ResMut<CardImageSet>>,
     mut state_events: MessageWriter<StateChangedEvent>,
+    mut images: ResMut<Assets<Image>>,
 ) {
     let Some(active) = active else { return };
     let active_id = active.0.id();
@@ -207,6 +208,9 @@ fn sync_card_image_set_with_active_theme(
     let Some(theme) = themes.get(&active.0) else {
         return;
     };
+    if theme.meta.pixel_art {
+        apply_nearest_sampler_to_theme_images(theme, &mut images);
+    }
     let Some(card_image_set) = card_image_set.as_deref_mut() else {
         return;
     };
@@ -214,6 +218,32 @@ fn sync_card_image_set_with_active_theme(
     state_events.write(StateChangedEvent);
 }
 
+/// Overrides the texture sampler on every face + back `Image` in
+/// `theme` to nearest-neighbor (point) sampling, so non-integer
+/// downscales preserve crisp pixel-grid edges instead of bilinear
+/// blur.
+///
+/// Called only for themes whose manifest sets `meta.pixel_art = true`.
+/// SVG-rasterised themes leave the field at its `false` default and
+/// keep Bevy's smooth-downscale sampler — pixel-art and SVG paths use
+/// different filters from the same loader pipeline because they
+/// optimise for different artwork tradeoffs.
+fn apply_nearest_sampler_to_theme_images(
+    theme: &CardTheme,
+    images: &mut Assets<Image>,
+) {
+    use bevy::image::{ImageSampler, ImageSamplerDescriptor};
+    let nearest = ImageSampler::Descriptor(ImageSamplerDescriptor::nearest());
+    for handle in theme.faces.values() {
+        if let Some(image) = images.get_mut(handle) {
+            image.sampler = nearest.clone();
+        }
+    }
+    if let Some(image) = images.get_mut(&theme.back) {
+        image.sampler = nearest;
+    }
+}
+
 /// Pure helper that copies the theme's image handles into the
 /// `[suit][rank]` face matrix and into the dedicated `theme_back`
 /// slot. Split out so it can be unit-tested without spinning up a
@@ -459,6 +489,7 @@ mod tests {
                 author: "test".into(),
                 version: "0".into(),
                 card_aspect: (2, 3),
+                pixel_art: false,
             },
             faces: HashMap::new(),
             back: Handle::default(),
@@ -723,6 +754,7 @@ mod tests {
                     author: "x".into(),
                     version: "x".into(),
                     card_aspect: (2, 3),
+                    pixel_art: false,
                 },
             }],
         });