Add Iroh P2P transport for private remote connectivity

[rueckwaerts]

Summary

Adds an optional Iroh QUIC P2P transport to electrs, allowing wallet software to connect directly to a home node from anywhere in the world — without TCP port forwarding, Tor, or VPN.

The user only needs to know the server's Iroh Node ID (a 64-char hex string). NAT traversal is handled automatically.

Motivation

Users running electrs at home (Start9, Umbrel, RaspiBlitz, bare metal) currently have no good option for remote access:

• Open router ports — security risk, not always possible
• Tor — slow, complex setup, not reliable on mobile
• VPN — requires a VPS or complex router config

This is a real barrier for anyone who wants to use their own node from outside their home network.

Iroh solves this cleanly: the relay is only used for the initial handshake (NAT traversal). Once connected, traffic flows peer-to-peer, end-to-end encrypted via QUIC/TLS. The relay never sees payload content.

Notably, Fedimint — a well-known Bitcoin privacy project — uses Iroh for exactly this reason: P2P connectivity between guardians without infrastructure overhead. This validates Iroh as a production-ready choice for Bitcoin-related software.

Changes

• src/iroh_listener.rs — new Iroh listener (accepts incoming QUIC connections, pipes to existing recv_loop)
• src/server.rs — pub fn recv_loop (no logic change, visibility only)
• src/lib.rs — iroh_listener module included behind iroh feature flag
• Cargo.toml — optional iroh feature: cargo build --release --features iroh

Configuration

Secret key path defaults to ~/.electrs/iroh_secret_key.bin, configurable via:

--iroh-secret-key-path <PATH>

The Node ID is printed to logs on startup — no file write required.

ALPN

b"electrs/electrum/0"

Testing

Tested end-to-end from outside home network, against electrs on Start9:

• Sparrow Wallet → Iroh relay → electrs: Connected to electrs/0.11.1 on protocol version 1.4 ✓
• Electrum → Iroh relay → electrs ✓

Client-side bridge: https://github.com/rueckwaerts/iroh-electrum-bridge
Sparrow integration PR: sparrowwallet/sparrow#2021
Electrum integration PR: spesmilo/electrum#10695

Feature Flag

Iroh support is fully opt-in and does not affect existing builds:

cargo build --release                          # unchanged
cargo build --release --features iroh          # with Iroh support