Fix migration guards and emergency controls

[rikkk-j]

Summary

• add claims-processor migration version validation and explicit migration sequencing
• add per-data-type pause/resume controls in oracle-verifier
• add configurable max products per pool/category in policy-engine
• add admin-approved LP emergency withdrawal in risk-pool

Issues

Closes #343
Closes #344
Closes #345
Closes #346

Tests

• cargo test -p parashield-claims-processor -p parashield-oracle-verifier -p parashield-policy-engine -p parashield-risk-pool (fails before contract tests: soroban-env-host v22.1.3 does not compile with ed25519-dalek 3.0.0 RNG trait bounds in this environment)

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