This commit finalizes the integration of Palladium Lightning with
the Palladium Core backend, making it trivial for developers to
spin up and test a local regtest network.
Key changes:
- Created [TESTING_PALLADIUM.md](cci:7://file:///home/davide/lightning-plm/TESTING_PALLADIUM.md:0:0-0:0) with comprehensive regtest instructions.
- Linked the new testing guide in the main [README.md](cci:7://file:///home/davide/lightning-plm/README.md:0:0-0:0).
- Ignored `/palladium-bin/` in `.gitignore` to allow developers to drop
backend binaries locally without accidentally committing them.
- Patched `contrib/startup_regtest.sh` to automatically detect and use
these local `palladiumd`/`palladium-cli` binaries if present, removing
the strict need to manually export environment variables.
- Fixed a false-positive timeout error in `startup_regtest.sh` when
parsing the absent `clnrest` plugin logs
