Verify the Lightning (LND) integration end-to-end

[himanshu64]

State: Lightning RPCs (createInvoice/payInvoice/lightningBalance) need a running LND; unverified without one.
Goal: stand up LND + open channels on regtest/signet and verify invoice creation, payment, and Lightning loan-repayment.
Where: himsha-node/src/lightning.rs, docs/lightning.md.