fix(engine): regenerate table backgrounds to flat Terminal palette

The post-Option-D screenshot showed Terminal cards correctly but
a green felt play surface — the chrome migration only retuned
in-engine constants, leaving the on-disk PNGs at
assets/backgrounds/bg_*.png as the legacy felt textures.

Adds solitaire_engine/examples/background_generator.rs following
the same regeneratable pattern as card_face_generator. Five solid
near-black variants from the base16-eighties palette:

- bg_0: #151515 (Terminal canonical, BG_PRIMARY)
- bg_1: #0a0a0a (BG_DEEPEST)
- bg_2: #1a1a1a (BG_ELEVATED — same as card face)
- bg_3: #121820 (slight cool tint)
- bg_4: #201812 (slight warm tint)

Per design-system.md the Terminal play surface is *flat* — no
felt, no gradient — so all 5 slots are pure solid colours. Each
PNG is 120 × 168 (matches the legacy tile size; spawn_background
stretches to window_size * 2.0 at runtime so source resolution
is immaterial). On-disk weight drops from ~16KB average to ~100
bytes per tile.

Run with: cargo run --example background_generator --release

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

solitaire_engine/examples/background_generator.rs

@@ -0,0 +1,72 @@
+//! Table-background generator — writes 5 Terminal-aesthetic solid-colour
+//! PNGs into `assets/backgrounds/`.
+//!
+//! Run with:
+//!
+//! ```sh
+//! cargo run --example background_generator --release
+//! ```
+//!
+//! The play-surface fill is `Settings::selected_background`-indexed
+//! into `assets/backgrounds/bg_{0..4}.png` by
+//! `table_plugin::load_background_images`. Per `design-system.md` the
+//! Terminal play surface is *flat* — no felt texture, no gradient —
+//! so all five slots are simple solid colours from the base16-eighties
+//! palette, each visually distinct enough for the picker but all on
+//! brand.
+//!
+//! Output is small (a 120 × 168 PNG with one solid colour compresses
+//! to ≈100 bytes) and stretched to `window_size * 2.0` at runtime by
+//! `table_plugin::spawn_background`, so the source resolution is
+//! immaterial — keeping it 120 × 168 preserves the legacy tile size.
+
+use std::path::PathBuf;
+use tiny_skia::{Color, IntSize, Pixmap};
+
+const TILE_W: u32 = 120;
+const TILE_H: u32 = 168;
+
+/// Five Terminal-palette play-surface variants. Slot 0 is the canonical
+/// design-system `#151515`; the others stay in the same near-black
+/// family so the picker offers choice without ever leaving the brand.
+const BACKGROUNDS: [(u8, u8, u8); 5] = [
+    (0x15, 0x15, 0x15), // 0 — Terminal canonical (#151515 BG_PRIMARY)
+    (0x0a, 0x0a, 0x0a), // 1 — deeper near-black (BG_DEEPEST)
+    (0x1a, 0x1a, 0x1a), // 2 — BG_ELEVATED (matches card face)
+    (0x12, 0x18, 0x20), // 3 — slight cool tint
+    (0x20, 0x18, 0x12), // 4 — slight warm tint
+];
+
+fn main() {
+    let dir = workspace_assets_dir().join("backgrounds");
+    std::fs::create_dir_all(&dir).expect("create backgrounds dir");
+
+    let size = IntSize::from_wh(TILE_W, TILE_H).expect("non-zero tile size");
+
+    for (idx, (r, g, b)) in BACKGROUNDS.iter().enumerate() {
+        let mut pixmap = Pixmap::new(size.width(), size.height()).expect("alloc pixmap");
+        pixmap.fill(Color::from_rgba8(*r, *g, *b, 0xff));
+
+        let path = dir.join(format!("bg_{idx}.png"));
+        pixmap
+            .save_png(&path)
+            .unwrap_or_else(|e| panic!("write {}: {e}", path.display()));
+    }
+
+    println!(
+        "Wrote 5 PNGs ({}×{} solid Terminal colours) to {}",
+        TILE_W,
+        TILE_H,
+        dir.display(),
+    );
+}
+
+/// Resolves the workspace-root `assets/` directory relative to the
+/// running example crate. `CARGO_MANIFEST_DIR` is the engine crate;
+/// its parent is the workspace root.
+fn workspace_assets_dir() -> PathBuf {
+    PathBuf::from(env!("CARGO_MANIFEST_DIR"))
+        .parent()
+        .expect("solitaire_engine crate has a workspace-root parent")
+        .join("assets")
+}