[documentation] Document Soroban contract upgrade path (PRD G8)

[yinkscss]

Context

PRD v0.2 G8 requires mainnet-ready contract patterns including a documented upgrade path.

Problem

No upgrade or migration documentation exists for contracts/waveflow-escrow beyond deploy script stub.

Proposed scope

• Add docs/contract-upgrade.md covering WASM upgrade procedure, state compatibility, maintainer auth requirements, and testnet validation steps
• Cross-link from README and scripts/deploy-contract.sh

Acceptance criteria

• Upgrade path documented with step-by-step maintainer actions
• Breaking change policy stated
• Linked from README Documentation section

References

• docs/PRD.md (G8, Technical Constraints)
• contracts/waveflow-escrow/
• scripts/deploy-contract.sh