Multi region replication

[gloskull]

Motivation

• Provide a dependency-free, deterministic representation of multi-region topology, failover planning, canary release gates, and DR test reporting to standardize DR decisions across services (issue Multi-Region Replication and Disaster Recovery Testing #91).
• Expose stable, off-chain-friendly types so monitoring and deployment tooling can evaluate the same gates without adding runtime dependencies.

Description

• Add a new replication module (src/replication/mod.rs) implementing RegionStatus, ReplicationTopology, deterministic failover selection (failover_plan()), CanaryAnalysis release gates, DisasterRecoveryTestReport, and ReplicationMetrics with SLO constants.
• Register the module from the crate root by adding pub mod replication; to src/lib.rs so consumers and tests can use it.
• Add integration tests (tests/replication_dr_test.rs) covering failover selection, stale/slow-region DR blocking, canary gates (security, success rate, latency), and combined RTO/RPO pass criteria.
• Add an operator-facing runbook (docs/runbooks/multi-region-dr.md) describing architecture, SLOs, blue-green/canary promotion flow, DR exercise steps, and monitoring/alert guidance.

Testing

• Ran cargo test --test replication_dr_test and the new replication tests passed (4 passed; 0 failed).
• Ran the full test suite with cargo test; the replication tests passed, but the overall run produced unrelated failures in existing pool_manager unit tests (17 failures) due to Soroban storage access from tests that require env.as_contract(); those failures are pre-existing and not caused by the replication module.

Closes #91

[JamesEjembi]

Merged directly into main with conflict resolution