nx_blob_store_mcu_test.nx

nx_blob_store_mcu_test.nx -- smoke for MCU-tier blob_store variant. Exercises: 1. Allocation + canary + initial state 2. Put + get roundtrip on a known sequence 3. DEDUP: same bytes twice -> same hash + count unchanged 4. Distinct bytes -> distinct hashes 5. has_blob positive + negative 6. SHA-256 determinism across two MCU stores 7. Bad-input gates (null bytes with len>0, negative len, null out_hash, null hash on get, null out_buf, undersized buffer) 8. MCU-specific OVERSIZED GATE: blob > 256 bytes -> NX_BLOB_MCU_TOO_BIG 9. CAPACITY GATE: put 4 distinct + try 5th -> NX_BLOB_MCU_FULL 10. TAMPER detection: canary stomp -> ops refuse 11. Empty-blob roundtrip + dedup 12. Sealed-enum verdict gate