feat(engine): drop shadows on cards with lifted state during drag

Cards previously read as flat stickers on the felt — no separation
cue, no sense the play surface had any depth. Each CardEntity now
spawns a CardShadow child sprite: neutral black at 25 % alpha, sized
to card_size + 4 px halo, offset (2, -3) and rendered at local z
-0.05 so it sits behind its card.

Cards in the active drag set switch to a lifted shadow: alpha 40 %,
offset (4, -6), padding (8, 8). update_card_shadows_on_drag runs
every Update and snaps each shadow to the right state based on
DragState membership — no lerp, no animation cost. The pure
card_shadow_params(is_dragged) helper is unit-tested for the four
parameter values.

resize_cards_in_place gains a third query for shadows so the
in-place resize keeps shadows cheap (no Sprite regeneration); the
shadow's current alpha is read to preserve idle vs lifted padding
across a resize. update_card_entity's despawn_related call is
followed by a fresh add_card_shadow_child so the shadow re-attaches
when the card is repainted (face flip, settings change, theme
swap). The pre-existing bulk drag-shadow under the whole lifted
stack is untouched — per-card shadows complement it.

All shadow values flow through eight new ui_theme tokens
(CARD_SHADOW_COLOR, alphas, offsets, paddings, local z) so the
visual is tunable in one place. Color is neutral black so the
shadows don't conflict with color-blind mode's red/blue suit tints.

Four new tests pin the contract: shadow params for idle and drag
states, every CardEntity spawns with exactly one CardShadow child,
and dragging shifts only the dragged shadow's offset while leaving
unrelated shadows on the idle offset.

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

solitaire_engine/src/ui_theme.rs

@@ -16,6 +16,7 @@
 //! changing the constant API.
 
 use bevy::color::Color;
+use bevy::math::Vec2;
 use bevy::prelude::Val;
 use solitaire_data::AnimSpeed;
 
@@ -119,6 +120,82 @@ pub const DROP_TARGET_OUTLINE_PX: f32 = 3.0;
 /// `bevy::ui`, while this is a 2D `Sprite` z coordinate.
 pub const Z_DROP_OVERLAY: f32 = 50.0;
 
+/// Background colour of the stock-pile remaining-count chip.
+///
+/// Reuses `BG_ELEVATED_HI` so the chip reads as one rung above the
+/// translucent stock pile marker without introducing a new palette
+/// value. The badge sits on the stock corner so the player knows how
+/// many cards remain before a recycle.
+pub const STOCK_BADGE_BG: Color = BG_ELEVATED_HI;
+
+/// Foreground (text) colour of the stock-pile remaining-count chip.
+///
+/// `ACCENT_PRIMARY` keeps the chip readable against the elevated
+/// purple background and matches the Balatro accent already used for
+/// other "look here" callouts.
+pub const STOCK_BADGE_FG: Color = ACCENT_PRIMARY;
+
+/// Sprite-space `Transform.z` for the stock-pile remaining-count chip.
+///
+/// Sits above the stock pile marker (`Z_PILE_MARKER` = `-1`) and any
+/// face-down stock cards (which start at `0`), but well below
+/// [`Z_DROP_OVERLAY`] (`50.0`) so the green drop-target wash always
+/// renders on top while a card is being dragged. Like `Z_DROP_OVERLAY`,
+/// this is a 2D `Sprite` z coordinate, not a `bevy::ui` `ZIndex`.
+pub const Z_STOCK_BADGE: f32 = 30.0;
+
+// ---------------------------------------------------------------------------
+// Card drop-shadow — the subtle dark halo painted beneath every card so the
+// play surface reads as physical instead of a flat collage of stickers. Idle
+// values are deliberately low-contrast (small offset, ~25% alpha) so resting
+// cards feel grounded without competing with focus rings or drop overlays.
+// Drag values are slightly stronger (further offset, ~40% alpha, larger
+// halo) so the dragged stack visually "lifts" off the felt.
+// ---------------------------------------------------------------------------
+
+/// RGB base for the per-card drop shadow. Always neutral black — never
+/// suit-tinted — so the shadow never carries colour information that a
+/// colour-blind player would rely on to identify a card. Alpha is applied
+/// separately via [`CARD_SHADOW_ALPHA_IDLE`] / [`CARD_SHADOW_ALPHA_DRAG`].
+pub const CARD_SHADOW_COLOR: Color = Color::srgb(0.0, 0.0, 0.0);
+
+/// Alpha for the resting-state card shadow. Low enough that 52 stacked
+/// shadows do not darken the felt into a uniform smear, high enough that
+/// each card reads as separated from the surface.
+pub const CARD_SHADOW_ALPHA_IDLE: f32 = 0.25;
+
+/// Alpha for the lifted/dragged card shadow. Stronger than the idle value
+/// so the dragged stack visibly "casts more shadow" while the player holds
+/// it above the table.
+pub const CARD_SHADOW_ALPHA_DRAG: f32 = 0.40;
+
+/// World-space pixel offset of the resting-state card shadow relative to
+/// its parent card centre. Down-and-right matches a soft top-left light
+/// source — the same convention used by the elevated-surface tones in the
+/// rest of the palette.
+pub const CARD_SHADOW_OFFSET_IDLE: Vec2 = Vec2::new(2.0, -3.0);
+
+/// World-space pixel offset of the lifted/dragged card shadow. Roughly
+/// double the idle offset so the parallax reads as "the card is further
+/// from the table".
+pub const CARD_SHADOW_OFFSET_DRAG: Vec2 = Vec2::new(4.0, -6.0);
+
+/// Padding in pixels added to each axis of the card size when sizing the
+/// resting-state shadow sprite. The shadow extends slightly past every
+/// edge of the card so the dark border reads as a halo rather than a
+/// matte rectangle behind the card.
+pub const CARD_SHADOW_PADDING_IDLE: Vec2 = Vec2::new(4.0, 4.0);
+
+/// Padding added to the card size when sizing the lifted/dragged shadow.
+/// A slightly larger halo at the drag state reinforces the "lifted off
+/// the felt" cue alongside the deeper offset and higher alpha.
+pub const CARD_SHADOW_PADDING_DRAG: Vec2 = Vec2::new(8.0, 8.0);
+
+/// Local `Transform.z` for the shadow child sprite, relative to its
+/// parent `CardEntity`. Slightly negative so the shadow always renders
+/// below the card itself even though it shares the parent's world z.
+pub const CARD_SHADOW_LOCAL_Z: f32 = -0.05;
+
 /// Subtle border — default popover, card, and idle button outline.
 pub const BORDER_SUBTLE: Color = Color::srgba(0.647, 0.549, 1.000, 0.12);