fix(pubsub): treat pending subscriptions as local interest

[marcus-pousette]

Summary

• make debounced subscribe(topic) intent explicit via pendingSubscriptions
• treat pending topics as local interest for strict PubSubData delivery checks
• include pending topics in subscription-overlap responses used by subscribe/request-subscriber handshakes
• eagerly initialize topic tracking on subscribe() and clean it up when a pending subscribe is cancelled before debounce

Why

subscribe() is debounced. Without explicit pending intent, there is a window where strict targeted delivery can be dropped and overlap responses under-report local interest.

Tests

• added regression: pending subscribe is tracked immediately and fully cleaned on cancel-before-debounce
• added regression: strict direct PubSubData is accepted while subscribe is pending
• verified with: pnpm --filter @peerbit/pubsub test (pass)

Scope

This PR intentionally includes only the pubsub pending-subscription behavior and tests (no shared-log/docs/tooling changes).