feat(engine): click-outside-to-dismiss for read-only modals

Adds a ScrimDismissible marker to ui_modal that opts a modal into
the standard "click outside the card to close" gesture. The new
dismiss_modal_on_scrim_click system fires on a left-mouse press
whose cursor falls on the scrim and outside every ModalCard, then
despawns the topmost dismissible scrim — Bevy's hierarchy despawn
cascades to the card and its children.

Marker design is opt-in per modal so destructive / state-mutating
modals (Settings saves on close, Onboarding requires explicit
acknowledgement, Pause / Forfeit / ConfirmNewGame need confirmed
intent) don't lose work to an accidental scrim click. Three
read-only modals opt in this round:

- Stats — informational; press S or click outside to dismiss.
- Achievements — read-only list.
- Help — keyboard reference.

Profile, Leaderboard, and Home will opt in the same way in a
follow-up; they were left out to keep this commit's scope tight.

The hit-test path uses each ModalCard's UiGlobalTransform +
ComputedNode bounding box so stacked modals close cleanly: the
topmost dismissible scrim is the only candidate per click. Tests
spawn synthetic ComputedNodes (with bevy::sprite::BorderRect for
the resolved-border slots Bevy's UI module re-exports) so the
geometry hit-tests deterministically without running the full UI
layout pipeline.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

solitaire_engine/src/achievement_plugin.rs

@@ -32,6 +32,7 @@ use crate::settings_plugin::{SettingsResource, SettingsStoragePath};
 use crate::stats_plugin::{StatsResource, StatsUpdate};
 use crate::ui_modal::{
     spawn_modal, spawn_modal_actions, spawn_modal_button, spawn_modal_header, ButtonVariant,
+    ScrimDismissible,
 };
 use crate::ui_theme::{
     ACCENT_PRIMARY, BORDER_SUBTLE, STATE_SUCCESS, TEXT_DISABLED, TEXT_PRIMARY, TEXT_SECONDARY,
@@ -473,7 +474,7 @@ fn spawn_achievements_screen(
         ..default()
     };
 
-    spawn_modal(commands, AchievementsScreen, Z_MODAL_PANEL, |card| {
+    let scrim = spawn_modal(commands, AchievementsScreen, Z_MODAL_PANEL, |card| {
         spawn_modal_header(card, header, font_res);
 
         // Scrollable body — the achievements list grows to ~19 rows which
@@ -577,6 +578,9 @@ fn spawn_achievements_screen(
             );
         });
     });
+    // Achievements is a read-only list — clicking the scrim outside
+    // the card dismisses alongside the existing A / Done paths.
+    commands.entity(scrim).insert(ScrimDismissible);
 }
 
 fn format_reward(reward: Reward) -> String {