feat(stats): spawn Prev/Next replay selector in the Stats overlay

Wire the long-dormant ReplayPrevButton / ReplaySelectorCaption /
ReplayNextButton / ReplaySelectorDetail spawn site that was missing
since v0.19.0. The click handler and repaint systems already existed;
this commit adds the actual UI nodes so players can step through all
stored replays (up to REPLAY_HISTORY_CAP) instead of always watching
the most recent win.

Also fix an assertion-on-constant clippy lint in the replay_overlay
dim-layer z-order test (const { assert!() } form required).

1282 tests passing.

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

solitaire_engine/src/replay_overlay.rs

@@ -3986,9 +3986,6 @@ mod tests {
     /// silently flip the intended stacking.
     #[test]
     fn dim_layer_z_is_below_replay_chrome() {
-        assert!(
-            Z_REPLAY_DIM < Z_REPLAY_OVERLAY,
-            "dim layer (z={Z_REPLAY_DIM}) must be below replay chrome (z={Z_REPLAY_OVERLAY})",
-        );
+        const { assert!(Z_REPLAY_DIM < Z_REPLAY_OVERLAY) }
     }
 }