feat(engine): UX iteration round — tooltip slider, streak fire, score breakdown

Three small UX improvements bundled because they share ui_theme token
edits.

Tooltip-delay slider in Settings → Gameplay
- Settings.tooltip_delay_secs (f32, #[serde(default)] = 0.5) tunable
  via "−" / "+" icon buttons next to a value readout. Range
  [TOOLTIP_DELAY_MIN_SECS, TOOLTIP_DELAY_MAX_SECS] = [0.0, 1.5] in
  TOOLTIP_DELAY_STEP_SECS (0.1) increments. "Instant" label when
  value is 0; "{n:.1} s" otherwise.
- ui_tooltip's hover-delay comparison reads from SettingsResource
  with MOTION_TOOLTIP_DELAY_SECS as the fallback when the resource
  is absent (test path). New tooltip_should_show(elapsed, delay)
  pure helper covers the boundary cases.
- adjust_tooltip_delay clamps; sanitized() carries the clamp through
  load. Five round-trip / default / legacy-deserialise tests.

Win-streak milestone fire animation
- New WinStreakMilestoneEvent { streak: u32 } fired from stats_plugin
  when win_streak_current crosses any of [3, 5, 10] (only the
  threshold crossing — not every subsequent win). HUD streak readout
  scale-pulses 1.0 → 1.20 → 1.0 over MOTION_STREAK_FLOURISH_SECS
  (0.6 s) on receipt; mirrors the foundation-flourish curve shape.
- Three threshold-crossing tests pin the firing contract.

Score-breakdown reveal on the win modal
- Win modal body replaces the single "Score: N" line with a
  per-component reveal: Base score, Time bonus (m:ss), No-undo
  bonus, Mode multiplier, separator, Total. Rows fade in over
  MOTION_SCORE_BREAKDOWN_FADE_SECS (0.12 s) staggered by
  MOTION_SCORE_BREAKDOWN_STAGGER_SECS (0.15 s) so the math reads as
  it animates. Skipped rows: zero time bonus, undo-tainted no-undo
  bonus, multiplier == 1.0.
- Honours AnimSpeed::Instant: rows spawn fully visible, no stagger.
- New ScoreBreakdown::compute helper sources base from
  GameWonEvent.score, time bonus from
  solitaire_core::scoring::compute_time_bonus, no-undo from a +25
  constant when undo_count == 0, mode multiplier from GameMode (Zen
  zeros the total). 9 new tests cover the math and the reveal
  cadence.

Test count net: +25 across the workspace (1007 → 1031).

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

solitaire_engine/src/events.rs

@@ -83,6 +83,26 @@ pub struct FoundationCompletedEvent {
     pub suit: Suit,
 }
 
+/// Fired by `StatsPlugin` when the player's `win_streak_current`
+/// crosses one of the milestone thresholds in
+/// [`crate::ui_theme::STREAK_MILESTONES`] (currently 3, 5, 10).
+///
+/// Fires only on the threshold crossing — i.e. when the previous
+/// streak was below the threshold and the post-win streak is at or
+/// above it — so subsequent wins past the highest milestone do not
+/// retrigger the flourish.
+///
+/// Drives the HUD streak-milestone flourish (a brief scale pulse on
+/// the score readout) and an informational toast. UI/audio cue only;
+/// not persisted, not synchronised.
+#[derive(Message, Debug, Clone, Copy)]
+pub struct WinStreakMilestoneEvent {
+    /// The new `win_streak_current` value at the moment the
+    /// threshold was crossed. Always equal to a value in
+    /// [`crate::ui_theme::STREAK_MILESTONES`].
+    pub streak: u32,
+}
+
 /// Fired when a card's face-up state changes during gameplay.
 #[derive(Message, Debug, Clone, Copy)]
 pub struct CardFlippedEvent(pub u32);