fix: survive tunnel death during sustained downloads

- Add frame rescue: drain dead tunnel send buffers and re-broadcast via live tunnels
- Add orphan queue: buffer rescued frames when no tunnels alive, flush on reconnect
- Add dedup gap timeout (3s): skip missing sequences instead of stalling forever
- Add FlushDedup on CLOSE: deliver buffered data before ending session
- Zero latency impact on normal path (mining shares unaffected)
- Sessions now survive simultaneous tunnel death instead of hanging 15-20s
- CI: skip slow integration tests in short mode

fix: add cmd/xplex source, fix gitignore, bundle examples in releases