test: kill act env warnings + remove debug console noise

Root cause: mock _notify cb fires during waitFor poll, but testing-library
asyncWrapper flips IS_REACT_ACT_ENVIRONMENT=false. Raw react act() then
warns 'not configured to support act' (146 warnings).

Fix: _notify sets globalThis.IS_REACT_ACT_ENVIRONMENT=true around act(),
restores after. setupTests sets flag globally. Both together = 0 warnings.

Also:
- Remove 4 debug console.log from App.js (encounter start/end, drag, add chars)
- Remove always-fires scenario summary console.log
- Add fastWaitFor helper (5ms poll) to testHelpers
- Swap scenario waitFor -> fastWaitFor

Tests: 77 pass / 0 fail / 0 warnings / 0 console noise.

src/tests/testHelpers.js

@@ -4,6 +4,9 @@ import { render, screen, fireEvent, waitFor, within } from '@testing-library/rea
 import App from '../App';
 import { MOCK_DB } from '../__mocks__/firebase/_mock-db';
 
+// fast waitFor: 5ms poll vs default 50ms. Cuts scenario ~8x.
+export const fastWaitFor = (cb, opts) => waitFor(cb, { interval: 5, timeout: 1000, ...opts });
+
 // Scoped container: the "Add Participants" section (avoids label clashes with CharacterManager).
 export function getParticipantForm() {
   const heading = screen.getByText('Add Participants');