style: cargo fmt under rustfmt 1.9 and gate formatting in CI

The repo was formatted under an older stable; rustfmt 1.9 (Rust 1.95)
wraps signatures and call sites differently, so every touched file was
picking up unrelated formatting hunks. One mechanical pass, and a
'cargo fmt --check' step in the test workflow (same pinned 1.95.0
toolchain) so drift can't accumulate again.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

solitaire_engine/src/onboarding_plugin.rs

@@ -27,6 +27,7 @@ use solitaire_data::{Settings, save_settings_to};
 
 use crate::font_plugin::FontResource;
 use crate::settings_plugin::{SettingsResource, SettingsStoragePath};
+use crate::splash_plugin::SplashRoot;
 use crate::ui_modal::{
     ButtonVariant, spawn_modal, spawn_modal_actions, spawn_modal_body_text, spawn_modal_button,
     spawn_modal_header,
@@ -36,7 +37,6 @@ use crate::ui_theme::{
     BORDER_SUBTLE, HighContrastBorder, RADIUS_SM, TEXT_PRIMARY, TYPE_BODY, TYPE_CAPTION,
     VAL_SPACE_1, VAL_SPACE_2, VAL_SPACE_3,
 };
-use crate::splash_plugin::SplashRoot;
 use crate::ui_theme::{TEXT_SECONDARY, Z_ONBOARDING};
 
 // ---------------------------------------------------------------------------