feat(core): unlock foundations — Foundation(u8) slots, suit derived from contents

Standard Klondike behaviour: any Ace can land in any empty foundation,
and that slot then claims the suit until the pile empties. The
previous PileType::Foundation(Suit) variant pre-assigned each of the
four foundations to a fixed suit ("C / D / H / S" placeholders) and
rejected mismatched Aces — non-standard and (per the smoke-test
feedback) confusing.

Replaces the variant payload with a slot index Foundation(u8) (0..=3)
and derives the claimed suit from the bottom card via a new
Pile::claimed_suit() method. The bottom card is, by construction,
the Ace that established the claim; using it directly eliminates an
entire class of "stuck claim after undo" bugs that a separate
claimed_suit field would have introduced.

can_place_on_foundation drops its suit parameter — the rule reduces
to "empty pile accepts any Ace; non-empty pile accepts the next
rank up of the bottom card's suit." Iteration sites across
input_plugin, cursor_plugin, selection_plugin, card_plugin,
auto_complete_plugin, game_plugin, layout, and hud_plugin all swap
the four-suit list for `(0..4u8).map(PileType::Foundation)`.

next_auto_complete_move now prefers a slot whose claimed_suit matches
the candidate card before falling back to the first empty slot for
an Ace — so the same suit consistently auto-targets the same slot
across the whole game, matching player expectations.

The HUD selection label and the hint toast read claimed_suit() and
fall back to "Foundation N" / "move to foundation" only when the
slot is empty. Empty foundation pile markers no longer render the
suit-letter children — they're plain translucent rectangles, matching
empty tableau placeholders.

Save-format invalidation: GameState gains a schema_version field
(serde-default to 1 for back-compat parsing of old files), the
constant is bumped to 2, and load_game_state_from rejects mismatched
schemas. Old in-progress saves silently fall through to "fresh game
on launch" — the user accepted this loss given the mechanic change.
Stats / progress / achievements / settings live in separate files,
contain no PileType data, and are unaffected.

9 new tests pin the contract:
- Pile::claimed_suit returns None for empty / non-foundation, Some
  for non-empty foundation
- Any Ace lands in the first empty foundation; successive Aces
  distribute across slots 0..3
- Claim drops when the slot is emptied via undo
- Auto-complete picks the slot with a matching claim, not the first
  empty slot
- A v1-format game_state.json is rejected; sibling stats save/load
  is unaffected

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

solitaire_engine/src/cursor_plugin.rs

@@ -13,7 +13,6 @@
 
 use bevy::prelude::*;
 use bevy::window::{CursorIcon, PrimaryWindow, SystemCursorIcon};
-use solitaire_core::card::Suit;
 use solitaire_core::game_state::{DrawMode, GameState};
 use solitaire_core::pile::PileType;
 use solitaire_core::rules::{can_place_on_foundation, can_place_on_tableau};
@@ -82,10 +81,10 @@ fn update_cursor_icon(
 fn cursor_over_draggable(cursor: Vec2, game: &GameState, layout: &Layout) -> bool {
     let piles = [
         PileType::Waste,
-        PileType::Foundation(Suit::Clubs),
-        PileType::Foundation(Suit::Diamonds),
-        PileType::Foundation(Suit::Hearts),
-        PileType::Foundation(Suit::Spades),
+        PileType::Foundation(0),
+        PileType::Foundation(1),
+        PileType::Foundation(2),
+        PileType::Foundation(3),
         PileType::Tableau(0),
         PileType::Tableau(1),
         PileType::Tableau(2),
@@ -158,12 +157,12 @@ fn update_drop_highlights(
 
     for (marker, mut sprite, _rch) in &mut markers {
         let valid = match &marker.0 {
-            PileType::Foundation(suit) => {
+            PileType::Foundation(slot) => {
                 if drag_count != 1 {
                     false
                 } else {
-                    let pile = game.0.piles.get(&PileType::Foundation(*suit));
-                    pile.is_some_and(|p| can_place_on_foundation(&bottom_card, p, *suit))
+                    let pile = game.0.piles.get(&PileType::Foundation(*slot));
+                    pile.is_some_and(|p| can_place_on_foundation(&bottom_card, p))
                 }
             }
             PileType::Tableau(idx) => {