fix(sang-eratosthenes): keep the grid usable for primes above 50

rippling was raised before the stagger loop but lowered only inside the
final timeout, so a prime with no multiple left on the 100-cell grid left
the flag raised and rejected every later click. Affected 10 of the 25
primes on the grid.

src/lib/lessons/sang-eratosthenes/grid-interaction.svelte.js

@@ -51,6 +51,14 @@ export function createGridState(buildAnnouncement) {
     markedPrimes = [...markedPrimes, n];
     const multiples = multiplesOf(n, 100);
 
+    // Every prime above 50 has no multiple left on a 100-cell grid. There is
+    // nothing to stagger, and entering ripple mode would lock the grid for
+    // good, because only the final timeout clears the flag.
+    if (multiples.length === 0) {
+      announcement = buildAnnouncement(n, multiples);
+      return;
+    }
+
     if (reducedMotion) {
       const next = new Map(crossings);
       for (const cell of multiples) {

src/lib/lessons/sang-eratosthenes/grid-interaction.test.js

@@ -0,0 +1,65 @@
+import { describe, it, expect } from 'vitest';
+import { createGridState } from './grid-interaction.svelte.js';
+
+/** Minimal announcement builder; the tests only care that it was called. */
+const build = (/** @type {number} */ p, /** @type {number[]} */ mults) =>
+  `${p}: ${mults.length}`;
+
+describe('createGridState — marking primes', () => {
+  it('a prime with no multiple left on the grid does not lock it', () => {
+    const grid = createGridState(build);
+
+    // 53 is prime and 2×53 = 106 is off the 100-cell grid, so there is
+    // nothing to cross out.
+    grid.handleCellActivate(53);
+    expect(grid.rippling).toBe(false);
+    expect(grid.markedPrimes).toContain(53);
+    expect(grid.announcement).toBe('53: 0');
+
+    // The grid must still accept the next prime.
+    grid.handleCellActivate(59);
+    expect(grid.markedPrimes).toContain(59);
+  });
+
+  it('every prime above 50 leaves the grid usable', () => {
+    const grid = createGridState(build);
+    for (const p of [53, 59, 61, 67, 71, 73, 79, 83, 89, 97]) {
+      grid.handleCellActivate(p);
+      expect(grid.rippling).toBe(false);
+    }
+    expect(grid.markedPrimes).toHaveLength(10);
+  });
+
+  it('a prime with multiples enters ripple mode and blocks further clicks', () => {
+    const grid = createGridState(build);
+    grid.handleCellActivate(7); // 14, 21, … are on the grid
+    expect(grid.rippling).toBe(true);
+
+    grid.handleCellActivate(11);
+    expect(grid.markedPrimes).not.toContain(11);
+  });
+
+  it('a composite is rejected and marks no prime', () => {
+    const grid = createGridState(build);
+    grid.handleCellActivate(9);
+    expect(grid.markedPrimes).toHaveLength(0);
+    expect(grid.shakeIndex).toBe(8);
+  });
+
+  it('the same prime cannot be marked twice', () => {
+    const grid = createGridState(build);
+    grid.handleCellActivate(53);
+    grid.handleCellActivate(53);
+    expect(grid.markedPrimes).toEqual([53]);
+  });
+
+  it('reset clears marks and unlocks the grid', () => {
+    const grid = createGridState(build);
+    grid.handleCellActivate(7);
+    expect(grid.rippling).toBe(true);
+    grid.handleReset();
+    expect(grid.rippling).toBe(false);
+    expect(grid.markedPrimes).toHaveLength(0);
+    expect(grid.announcement).toBe('');
+  });
+});