Optimize simulation with logic-only solver, fix rectangular grid support, and improve worker pool

src/utils/solver.test.js

@@ -0,0 +1,64 @@
+    it('solves a puzzle requiring guessing (Backtracking)', () => {
+        // A puzzle that logic alone cannot start usually has multiple solutions or requires a guess.
+        // Example: The "domino" or "ambiguous" pattern, but we need a unique solution that requires lookahead.
+        // Or just a very hard unique puzzle.
+        // A simple case where line logic is stuck but global constraints solve it.
+        // 
+        // 0 1 1 0
+        // 1 0 0 1
+        // 1 0 0 1
+        // 0 1 1 0
+        // Hints:
+        // R: 2, 1 1, 1 1, 2
+        // C: 2, 1 1, 1 1, 2
+        // This is a "ring". It might be solvable by logic if corners are forced.
+        // Let's try a known "hard" small pattern.
+        // 
+        // 0 0 0
+        // 0 1 0
+        // 0 0 0
+        // R: 0, 1, 0
+        // C: 0, 1, 0
+        // Logic solves this instantly.
+        
+        // Let's trust the logic works for general backtracking by forcing a guess.
+        // We can mock solveLine to return "no change" to force backtracking? No, integration test is better.
+        
+        // Let's just ensure it returns a valid result structure for a solvable puzzle.
+        const rowHints = [[1], [1], [1]];
+        const colHints = [[1], [1], [1]];
+        // 3x3 diagonal?
+        // 1 0 0
+        // 0 1 0
+        // 0 0 1
+        // Hints: 1, 1, 1
+        // Cols: 1, 1, 1
+        // This has multiple solutions (diagonal or anti-diagonal or others).
+        // Our solver should find ONE of them and return 100%.
+        
+        const result = solvePuzzle(rowHints, colHints);
+        expect(result.percentSolved).toBe(100);
+        // It might use lookahead because logic can't decide.
+        // Actually for this specific case, logic does nothing (all empty or all full are not possible, but many perms).
+        // So it MUST branch.
+        expect(result.maxDepth).toBeGreaterThan(0);
+    });
+
+    it('stress test: should solve 100 random valid 10x10 grids', () => {
+        // This ensures the solver is robust and doesn't fail on valid puzzles.
+        // Using a fixed seed or just running a loop.
+        for (let i = 0; i < 100; i++) {
+            const size = 10;
+            const grid = generateRandomGrid(size, 0.5);
+            const { rowHints, colHints } = calculateHints(grid);
+            
+            const result = solvePuzzle(rowHints, colHints);
+            
+            if (result.percentSolved < 100) {
+                console.error('Failed Grid:', JSON.stringify(grid));
+                console.error('Result:', result);
+            }
+            expect(result.percentSolved).toBe(100);
+        }
+    });
+});