feat(opsce): add transfer validation rules with min/max limits and blocked address checks

[phertyameen]

Summary

Implements contracts/opsce/src/transfer_rules.rs - a validate_transfer guard and supporting admin functions called at the start of every transfer execution path.

Changes

• validate_transfer(env, from, to, asset_id, amount) - rejects self-transfers, blocked recipients, and out-of-range amounts
• set_transfer_limits(env, caller, asset_id, min, max) - admin-only; configures per-asset min/max thresholds
• block_address(env, caller, address) - admin-only; adds an address to the persistent blocked list
• Transfers with no limits configured skip amount checks entirely
• Module exported from opsce/src/lib.rs and consumed in multisig_transfer/src/rules.rs

Testing

All 8 unit tests pass via:

cargo test -p opsce

Covers: happy path, self-transfer, below minimum, above maximum, blocked recipient, no limits set, invalid limit config, boundary values (exact min/max).

Closes #827

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