Miden node persistence

Shared protobuf codecs for structured SQLite values and node-owned binary files. Raw database keys, execution proofs, and cryptographic encodings retain their formats.

Native serialization remains at these boundaries:

• SQLite keys and indexed words use fixed-width bytes for joins and cursor ordering.
• The proven_tip file stores one four-byte little-endian block number.
• Execution-proof files and proof fields retain the proof library encoding.
• Sealed transaction-input submissions use the existing RPC plaintext encoding.
• Keys, signatures, encrypted record keys, associated data and DKG files use their cryptographic library encodings.
• Upstream AccountFile handles account files. Its format is already protobuf.