fix(engine): hit-test face-down fan offset matches sprite layout

Smoke-test report: the user could only initiate a drag from the bottom
strip of a tableau card, not its visible face. Root cause was a fan-
step mismatch between rendering and hit-testing.

card_plugin::card_positions steps face-down cards by
TABLEAU_FACEDOWN_FAN_FRAC (0.12) and face-up cards by TABLEAU_FAN_FRAC
(0.25), so a column with 6 face-down + 1 face-up at the bottom
renders the face-up card at base.y - 0.72 * card_h. input_plugin's
card_position used a uniform 0.25 step for every position, computing
the same card's hit-test centre as base.y - 1.5 * card_h — almost a
full card height below the visible sprite. The hit-test AABB and the
sprite AABB overlapped only over the bottom 0.61 * card_h, which
matches the user's observation that only the bottom of the card
responds to clicks.

card_position now mirrors card_plugin's exact logic: walk the pile's
preceding cards and step by TABLEAU_FACEDOWN_FAN_FRAC for face-down,
TABLEAU_FAN_FRAC for face-up. TABLEAU_FACEDOWN_FAN_FRAC is now public
for the same reason TABLEAU_FAN_FRAC already was — the renderer and
the hit-tester have to agree by construction or this regression
returns.

Updates the existing find_draggable_skips_face_down_cards test that
relied on the old uniform-fan geometry, and adds
find_draggable_hits_face_up_card_with_face_down_cards_above_it as a
regression test that fails without this fix.

The during-drag rendering and pile_drop_rect still use the uniform
TABLEAU_FAN_FRAC because the cards being dragged are guaranteed
face-up, and a slightly oversized drop target reads as forgiving
rather than wrong. Those call sites are intentionally untouched.

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

solitaire_engine/src/card_plugin.rs

@@ -34,7 +34,14 @@ use crate::table_plugin::PileMarker;
 pub const TABLEAU_FAN_FRAC: f32 = 0.25;
 
 /// Tighter fan for face-down cards in the tableau — just enough to show the stack.
-const TABLEAU_FACEDOWN_FAN_FRAC: f32 = 0.12;
+/// Per-card vertical step for face-down tableau cards, as a fraction of
+/// card height. Smaller than [`TABLEAU_FAN_FRAC`] because face-down cards
+/// don't need their full body shown — only the back-pattern strip is
+/// visible. Public so `input_plugin` can mirror the exact sprite layout
+/// when hit-testing tableau columns; any drift between this and the
+/// renderer creates a visible offset between the card face and where
+/// clicks land.
+pub const TABLEAU_FACEDOWN_FAN_FRAC: f32 = 0.12;
 
 /// Fraction of card height used as a tiny offset between stacked cards in
 /// non-tableau piles, so stacking is visible.