Fix listener snapshots before singular event handlers

[er-s-an]

Summary

Capture the event listener snapshot before invoking the singular on* handler. This makes mutations from that handler obey the same documented next-dispatch behavior as mutations from addEventListener listeners.

Previously, an on* handler that added a listener caused that new listener to receive the event already in progress. Removing a listener from the handler caused it to miss the current event. Both contradicted EventDispatcher.dispatch()'s documented snapshot contract. This affects the shared notification fan-out used by App and AppBridge.

The fix preserves handler-first invocation and listener insertion order. The two regression tests verify both the current and subsequent dispatch.

Verification

• New regression tests against unchanged upstream: 2 failures, 4 existing tests passed.
• After fix: 247 SDK tests passed, 0 failed (bun test ./src/*.test.ts ./src/server/*.test.ts).
• tsc --noEmit passed.
• Prettier check of both changed files passed.
• git diff --check passed.
• Full build was attempted but blocked by this execution environment denying the tsx IPC socket (listen EPERM); no successful build or browser/E2E run is claimed.
• Running the broader bun test src also discovered a PDF example test without its workspace dependency @cantoo/pdf-lib; the scoped SDK suite above passes.

AI assistance was used for diagnosis, implementation and test preparation. The change was checked against fresh local regression results; no remote model APIs were needed.