Phase 14: LLM-grounded encounter-balancing tip

- src/lib/assistant/prompts.ts: balanceSuggestionSchema + buildBalancePrompt
  (system message leads with the grounding constraint; restricts choices to the
  provided compendium candidates).
- useEncounterAdvisor hook: builds context → picks system-correct candidates →
  asks the LLM (when configured) for a structured pick, validates + filters names
  to the candidate set, else falls back to the deterministic buildSuggestedEncounter.
  Apply adds the creatures transactionally via encountersRepo.mutate.
- EncounterTipCard in the combat tracker: leads with the detected difficulty
  tendency (or the current difficulty), offers a one-click grounded suggestion
  (AI or deterministic) with propose→confirm Apply.

prompts unit tests + e2e for both the deterministic apply flow and the AI path
(stubbed provider); the AI test also confirms candidate-grounding (only
shortlisted creatures survive).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

e2e/assistant-llm.spec.ts

@@ -0,0 +1,73 @@
+import { test, expect } from '@playwright/test';
+
+test.beforeEach(async ({ page }) => {
+  await page.goto('/');
+  await page.evaluate(async () => {
+    indexedDB.deleteDatabase('ttrpg-manager');
+    localStorage.clear();
+  });
+  await page.reload();
+});
+
+test('encounter tip suggests and applies a deterministic balance fix', async ({ page }) => {
+  // Sample campaign: 2 level-3 PCs + a "Goblin Ambush" with 3 goblins.
+  await page.getByLabel('Settings').click();
+  await page.getByRole('button', { name: 'Load sample campaign' }).click();
+  await expect(page.getByRole('heading', { name: 'Sample: Lost Mine' })).toBeVisible();
+
+  // Open the seeded encounter in the combat tracker.
+  await page.getByLabel('Primary').getByRole('link', { name: 'Combat' }).click();
+  await page.getByText('Goblin Ambush').click();
+
+  // The contextual balance tip is present (LLM off → deterministic path).
+  const tip = page.getByTestId('encounter-tip');
+  await expect(tip).toBeVisible();
+
+  // Each combatant row has a "Dmg" button — use it to count combatants.
+  const hpControls = page.getByRole('button', { name: 'Dmg' });
+  const before = await hpControls.count();
+
+  await tip.getByRole('button', { name: 'Suggest a fix' }).click();
+  await expect(tip.getByRole('button', { name: 'Add to encounter' })).toBeVisible();
+  await tip.getByRole('button', { name: 'Add to encounter' }).click();
+
+  // Suggestion closes and at least one combatant was added.
+  await expect(tip.getByRole('button', { name: 'Add to encounter' })).toBeHidden();
+  await expect.poll(async () => hpControls.count()).toBeGreaterThan(before);
+});
+
+test('encounter tip uses the AI path when a provider is configured', async ({ page }) => {
+  // Stub the Anthropic endpoint with a canned structured response.
+  await page.route('**/v1/messages', async (route) => {
+    await route.fulfill({
+      status: 200,
+      contentType: 'application/json',
+      body: JSON.stringify({
+        content: [{ type: 'text', text: JSON.stringify({
+          reasoning: 'An ettin raises the threat to match the party.',
+          add: [{ name: 'Ettin', count: 1 }],
+          targetDifficulty: 'Hard',
+        }) }],
+      }),
+    });
+  });
+
+  await page.getByLabel('Settings').click();
+  await page.getByRole('button', { name: 'Load sample campaign' }).click();
+  await expect(page.getByRole('heading', { name: 'Sample: Lost Mine' })).toBeVisible();
+
+  // Enable the assistant with a dummy key (network is stubbed).
+  await page.getByLabel('Settings').click();
+  await page.getByLabel('Enable AI assistant').check();
+  await page.getByLabel('API key', { exact: true }).fill('sk-stub');
+
+  await page.getByLabel('Primary').getByRole('link', { name: 'Combat' }).click();
+  await page.getByText('Goblin Ambush').click();
+
+  const tip = page.getByTestId('encounter-tip');
+  await tip.getByRole('button', { name: 'Suggest a fix (AI)' }).click();
+  // The canned AI reasoning + the "AI suggestion" label render.
+  await expect(tip.getByText('AI suggestion')).toBeVisible();
+  await expect(tip.getByText('An ettin raises the threat to match the party.')).toBeVisible();
+  await expect(tip.getByText('1× Ettin')).toBeVisible();
+});