Add buildShieldRequest shield note serializer

[mattgle]

Adds the encrypt/serialize direction for shield notes: buildShieldRequest, a standalone builder that turns a ShieldNote into the { preimage, ciphertext } struct the RailgunSmartWallet contract expects.

The note random is encrypted with AES-256-GCM under the ECDH shared key of the shield private key and the receiver's viewing public key; the receiver's viewing public key is encrypted with AES-256-CTR under the shield private key. Both are packed into the contract's three 32-byte encryptedBundle slots plus shieldKey, so a built request decrypts cleanly back through ShieldNote.fromShieldCommitment.

It is a standalone function rather than a ShieldNote method, keeping the note a pure data model, and the serializer has no node: imports so it stays browser-safe.

Testss:

• Round-trip: built request through fromShieldCommitment recovers random, value, npk, token
• bundle[2] CTR-decrypts back to the receiver viewing public key
• Non-recipient decryption returns null without throwing
• Byte-identical golden fixture with a pinned IV

[bhflm]

lgtm