Add testnet4 compose sample namespaces

[joelklabo]

Summary

• add new compose sample namespaces for testnet4 while preserving existing testnet3 samples:
  • compose/env-sample/lndtn4
  • compose/env-sample/clntn4
• wire testnet4 network settings in the new sample files:
  • NETWORK='testnet4' in compose/app env files
  • testnet4=1 in Bitcoin Core configs
  • bitcoin.testnet4=1 in LND config
  • network=testnet4 in CLN config
  • testnet4 macaroon/CLN paths and RPC defaults
• update docs and aliases to advertise the new namespaces and add tn4* operator aliases

Why

• keeps testnet3 compatibility available for transition periods
• provides a ready-to-copy testnet4 stack namespace without breaking existing testnet3 samples

Validation

• git diff --check
• grep assertions for testnet4 values in all new namespace configs (NETWORK, Bitcoin Core, LND, CLN)

Related: RoboSats/robosats#1843