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/setupTests.js

@@ -23,6 +23,10 @@ console.warn = (...args) => {
 };
 import { resetMockDb } from './__mocks__/firebase/_mock-db';
 
+// Tell React this is an act environment. Without it, every act() call
+// emits 'current testing environment is not configured to support act'.
+globalThis.IS_REACT_ACT_ENVIRONMENT = true;
+
 // polyfill crypto.randomUUID for jsdom (used by generateId in App.js).
 if (!global.crypto) global.crypto = {};
 if (!global.crypto.randomUUID) {