BQIP-0004: Witness & L2 integration (planning + migration)

Draft added in docs/spec/BQIP-0004.md.\n\nAction items:\n- [ ] Finalize Witness layout + witness_root rules\n- [ ] Plan Transaction v2 + header v2 migration\n- [ ] Define fee/weight params (alpha,beta)\n- [ ] P2P inv/relay policy changes\n- [ ] Testnet activation plan