Add synchronous browser storage primitives

[Dzejkop]

Add synchronous Rust storage components for browser hosts and share SQLite connection configuration between encrypted vaults and plaintext blob databases.

• Add a software keystore backed by a host-supplied secret and a SQLite atomic blob store, with UniFFI constructors for both.
• Open and close the blob database connection on every operation. The database stores already-sealed blobs.
• Extract common SQLite configuration and plaintext opening alongside the encryption-specific code, including OPFS SAH pool support.
• Keep generic storage primitives in walletkit-db and the UniFFI adapters in walletkit-core.

This PR sits above #522 and below #523. Native primitive tests live alongside the implementations; browser integration coverage remains in #523 with its browser test harness. The split preserves the previously tested combined tree.