Add indicator and strategy interfaces

Introduce the typed interfaces that form the foundation of the
strategy framework: Indicator<T> for market analysis and Strategy
with Signal types for trading decisions.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/indicator.ts

@@ -0,0 +1,6 @@
+import { Alpaca } from "./alpaca";
+
+export interface Indicator<T> {
+    name: string;
+    evaluate(alpaca: Alpaca): Promise<T>;
+}