fix(engine): fill tableau fan to viewport on all aspect ratios

On a near-square viewport (e.g. an unfolded Galaxy Fold) a fresh deal left
the bottom ~40% of the screen empty: the dynamic fan (update_tableau_fan_frac)
measured only face-up column depth and returned early at a fresh deal (face-up
depth 1), so it never spread the tableau, and the deep face-down stacks were
ignored. The cold-start deal also never fired StateChangedEvent, and on Android
the safe-area-inset resize (frames 1-3) reset the fan to compute_layout's
sparse worst-case value, so even the post-move fill was wiped.

Move the fill into layout::apply_dynamic_tableau_fan, driven by each column's
TOTAL weighted depth (face-down cards count, scaled by the face-down/face-up
step ratio) so the deepest column fills the available height. Run it in three
places so every path stays filled: PostStartup (cold-start deal), on
StateChangedEvent (moves), and inside on_window_resized after compute_layout
(safe-area resize + fold/unfold). MAX_DYNAMIC_FAN_FRAC caps the spread so a
near-empty column keeps readable overlap; TABLEAU_FAN_FRAC floors it. Deeper
columns drive the fraction down so everything still fits — no overflow.
card_position/card_positions read the same fractions, so hit-testing stays
in sync.

Adds regression tests: cold-start deal fills the fan, and a resize re-fills it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

solitaire_engine/src/layout.rs

@@ -7,6 +7,7 @@ use std::collections::HashMap;
 
 use bevy::math::Vec2;
 use bevy::prelude::{Resource, SystemSet};
+use solitaire_core::game_state::GameState;
 use solitaire_core::{Foundation, KlondikePile, Tableau};
 
 /// Schedule labels for layout-related systems so cross-plugin ordering is
@@ -91,6 +92,15 @@ const TABLEAU_FACEDOWN_FAN_FRAC: f32 = 0.14;
 /// this column inside the visible window.
 const MAX_TABLEAU_CARDS: f32 = 13.0;
 
+/// Upper bound for the dynamic tableau fan step (fraction of card height) chosen
+/// by [`apply_dynamic_tableau_fan`]. The fan is spread to fill the available
+/// height, but a near-empty column has tiny demand, so without a cap its few
+/// cards would fling far apart on a tall viewport. At 0.6 the face-up cards keep
+/// clear overlap (a readable stack) while still filling most of a near-square /
+/// unfolded-foldable screen. Tunable purely for feel — no effect on correctness
+/// or hit-testing.
+pub(crate) const MAX_DYNAMIC_FAN_FRAC: f32 = 0.6;
+
 /// Vertical pixel band reserved at the top of the play area for the HUD
 /// (action buttons, Score / Moves / Timer readouts). The card grid starts
 /// below this band so the HUD doesn't bleed into the play surface.
@@ -309,6 +319,76 @@ pub fn compute_layout(
     }
 }
 
+/// Spread the tableau fan so the deepest column fills the available vertical
+/// height for the *current* deal, mutating `layout.tableau_fan_frac` and its
+/// face-down companion in place.
+///
+/// `compute_layout` is pure geometry and sizes the fan for a worst-case 13-card
+/// column, so early in a game (shallow columns) a tall or near-square viewport
+/// — e.g. an unfolded foldable — is left with a large empty band below the
+/// tableau. This refines the fan once the actual deal is known.
+///
+/// Depth is measured across *all* cards in a column, each face-down card
+/// weighted by the fixed face-down/face-up step ratio. Counting the face-down
+/// portion (not just the face-up tail) is what fills the lower screen on a fresh
+/// deal, where the deepest column is several face-down cards under one face-up
+/// one. Deeper columns drive the fraction down so everything still fits;
+/// [`TABLEAU_FAN_FRAC`] floors it and [`MAX_DYNAMIC_FAN_FRAC`] caps it.
+///
+/// Called from card-sync at startup and on every `StateChangedEvent`, and from
+/// the resize pipeline after `compute_layout`, so the cold-start deal, ongoing
+/// play, and fold/unfold all stay filled. `card_position` / `card_positions`
+/// read the same fractions, so rendering and hit-testing remain in sync.
+pub(crate) fn apply_dynamic_tableau_fan(game: &GameState, layout: &mut Layout) {
+    let card_h = layout.card_size.y;
+    let avail = layout.available_tableau_height;
+    if card_h <= 0.0 {
+        return;
+    }
+
+    let facedown_ratio = TABLEAU_FACEDOWN_FAN_FRAC / TABLEAU_FAN_FRAC;
+
+    // "Step demand" of a column: the vertical offset of its bottom card from its
+    // top card, in units of the face-up fan step. Every card except the last
+    // contributes one step, weighted down to `facedown_ratio` while face-down.
+    let max_demand = [
+        Tableau::Tableau1,
+        Tableau::Tableau2,
+        Tableau::Tableau3,
+        Tableau::Tableau4,
+        Tableau::Tableau5,
+        Tableau::Tableau6,
+        Tableau::Tableau7,
+    ]
+    .into_iter()
+    .map(|tableau| {
+        let pile = game.pile(KlondikePile::Tableau(tableau));
+        let steps = pile.len().saturating_sub(1);
+        pile.iter()
+            .take(steps)
+            .map(|(_, face_up)| if *face_up { 1.0 } else { facedown_ratio })
+            .sum::<f32>()
+    })
+    .fold(0.0_f32, f32::max);
+
+    // No fannable column (every tableau pile has ≤ 1 card) — leave the fractions
+    // at the values compute_layout set.
+    if max_demand <= 0.0 {
+        return;
+    }
+
+    let ideal = avail / (max_demand * card_h);
+    let new_frac = ideal.clamp(TABLEAU_FAN_FRAC, MAX_DYNAMIC_FAN_FRAC);
+    let new_facedown_frac = new_frac * facedown_ratio;
+
+    if (layout.tableau_fan_frac - new_frac).abs() > 1e-4 {
+        layout.tableau_fan_frac = new_frac;
+    }
+    if (layout.tableau_facedown_fan_frac - new_facedown_frac).abs() > 1e-4 {
+        layout.tableau_facedown_fan_frac = new_facedown_frac;
+    }
+}
+
 /// Bevy resource wrapping the current `Layout`. Recomputed on `WindowResized`.
 #[derive(Resource, Debug, Clone)]
 pub struct LayoutResource(pub Layout);