feat: Add blockchain architecture and test plan docs

- BLOCKCHAIN_DECISION.md: Cosmos SDK recommendation for Synapse Chain
- BLOCKCHAIN_PHOTON_INTEGRATION.md: Architecture for Photon-Core + Blockchain integration
- OWN_CHAIN_SPEC.md: Complete blockchain spec (SynapseVM, tokenomics, PoUW)
- PHOTON_BLOCKCHAIN_TEST_PLAN.md: Unit, integration, and simulation tests

Part of iberi22/synapse-protocol architecture work.

Author: SWAL Agent