Validate a serialized transaction against configured policy rules (allowed/blocked programs & instructions, signer/instruction constraints, CPI detection, flagged addresses, rate limits, simulation-based limits). Returns allow or block with human‑readable reasons.

When to use:
Call this before asking the user to sign or before submitting a transaction, to enforce guardrails and block unexpected behavior (non-whitelisted instructions/programs, suspicious flows, high-value movements, etc.)

Common reason codes (examples):
NOT_WHITELIST_PROGRAM, NOT_WHITELIST_INSTRUCTION, INVALID_INSTRUCTION_COUNT, MULTIPLE_SIGNERS_NOT_ALLOWED, FORBIDDEN_INSTRUCTION_COMBINATION, CPI_DETECTED, FLAGGED_ADDRESS_DETECTED, ASSET_VALUE_TOO_HIGH, SIMULATION_FAILED