Refactor peer connection state machine into separate files

Split the monolithic peer.ts file into a modular state-based architecture:
- Created separate files for each state class (idle, creating-offer, waiting-for-answer, answering, exchanging-ice, connected, failed, closed)
- Extracted shared types into types.ts
- Extracted base PeerState class into state.ts
- Updated peer/index.ts to import state classes instead of defining them inline
- Made close() method async to support dynamic imports and avoid circular dependencies
- Used dynamic imports in state transitions to prevent circular dependency issues

This improves code organization, maintainability, and makes each state's logic easier to understand and test.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/peer/closed-state.ts

@@ -0,0 +1,12 @@
+import { PeerState } from './state.js';
+
+/**
+ * Closed state - connection has been terminated
+ */
+export class ClosedState extends PeerState {
+  get name() { return 'closed'; }
+
+  cleanup(): void {
+    this.peer.pc.close();
+  }
+}