Add backtesting entry point for offline strategy testing

Introduces a BacktestClient that replays historical 1-min bars against the
existing strategy code, plus a CLI entry point (npm run backtest) that fetches
bars from Alpaca and runs the bot over a date range. Makes wait() pluggable
so the backtest resolves delays instantly while advancing simulated time.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/trading.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
-import { printAsset, accountBalance, waitForNextOpen } from './trading';
+import { printAsset, accountBalance, waitForNextOpen, wait, setWaitFn, resetWaitFn } from './trading';
 import type { Alpaca } from './alpaca';
 
 function mockAlpaca(overrides: Partial<Alpaca> = {}): Alpaca {
@@ -25,6 +25,7 @@ beforeEach(() => {
 afterEach(() => {
     vi.useRealTimers();
     vi.restoreAllMocks();
+    resetWaitFn();
 });
 
 describe('printAsset', () => {
@@ -82,3 +83,26 @@ describe('waitForNextOpen', () => {
     });
 });
 
+describe('setWaitFn / resetWaitFn', () => {
+    it('uses custom wait function when set', async () => {
+        const calls: number[] = [];
+        setWaitFn(async (ms) => { calls.push(ms); });
+
+        await wait(5000);
+        await wait(3000);
+
+        expect(calls).toEqual([5000, 3000]);
+    });
+
+    it('restores default wait after resetWaitFn', async () => {
+        setWaitFn(async () => {});
+        resetWaitFn();
+
+        // After reset, wait should use real setTimeout again
+        const promise = wait(1000);
+        await vi.advanceTimersByTimeAsync(1000);
+        await promise;
+        // If we got here without hanging, default wait is restored
+    });
+});
+