feat(client): local-only phantom-layer store

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

packages/client/src/lib/stores/phantoms.svelte.ts

@@ -0,0 +1,71 @@
+import {
+  opponentStartPosition,
+  deserializePhantoms,
+  type Color,
+  type Piece,
+  type PieceType,
+  type Square,
+} from '@blind-chess/shared';
+
+/**
+ * Client-LOCAL store for the player's phantom opponent-model layer.
+ * This data NEVER reaches the server — it is the player's private guess.
+ * Do not read this store in any `send`/`commit` path.
+ */
+function makeStore() {
+  const state = $state<{ phantoms: Partial<Record<Square, Piece>> }>({ phantoms: {} });
+  let gameId: string | null = null;
+  let oppColor: Color = 'b';
+
+  function key(id: string) { return `bc:phantoms:${id}`; }
+
+  function persist() {
+    if (gameId) localStorage.setItem(key(gameId), JSON.stringify(state.phantoms));
+  }
+
+  /** Load (or first-time seed) the phantom layer for a blind game. */
+  function loadForGame(id: string, you: Color) {
+    gameId = id;
+    oppColor = you === 'w' ? 'b' : 'w';
+    const raw = localStorage.getItem(key(id));
+    if (raw === null) {
+      // First load — seed with the opponent's starting army.
+      state.phantoms = opponentStartPosition(oppColor);
+      persist();
+    } else {
+      state.phantoms = deserializePhantoms(raw);
+    }
+  }
+
+  function place(sq: Square, type: PieceType) {
+    state.phantoms = { ...state.phantoms, [sq]: { color: oppColor, type } };
+    persist();
+  }
+
+  function move(from: Square, to: Square) {
+    const p = state.phantoms[from];
+    if (!p) return;
+    const next = { ...state.phantoms };
+    delete next[from];
+    next[to] = p;
+    state.phantoms = next;
+    persist();
+  }
+
+  function remove(sq: Square) {
+    const next = { ...state.phantoms };
+    delete next[sq];
+    state.phantoms = next;
+    persist();
+  }
+
+  /** Drop the layer when a game ends — avoids unbounded localStorage growth. */
+  function clearForGame(id: string) {
+    localStorage.removeItem(key(id));
+    if (gameId === id) { state.phantoms = {}; gameId = null; }
+  }
+
+  return { state, loadForGame, place, move, remove, clearForGame };
+}
+
+export const phantoms = makeStore();