feat: add NAT-traversing P2P file sharing example

[GautamBytes]

What was wrong?

The repository lacked a practical, runnable example demonstrating serverless, peer-to-peer file sharing across NATs with local peer persistence, as requested in the product-innovation bounty.

Issue #877

How was it fixed?

Added a lightweight, serverless file-sharing application under examples/file-share/ that fulfills the exact requirements of the issue.

Summary of approach.

• NAT Traversal: Leveraged the library's built-in capabilities by initializing the host with enable_upnp=True. The CLI visually splits Local IPs and Public IPs on startup to prove hole-punching success.

• Peer Persistence: Implemented a lightweight peerstore alternative using a local friends.json file. This acts as a "Contacts List," allowing the user to persist and instantly reconnect to known peers across restarts without needing to rely on external DHT or bootstrap nodes.

• Stream Protocol: Implemented a custom, chunk-based binary stream protocol (/file-share/1.0.0) over trio to transfer file metadata (name, size) and safely stream file data directly to disk without bloating RAM.

To-Do

• Clean up commit history
• Add or update documentation related to these changes
• Add entry to the release notes

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