Add ShieldNote.create factory for shield origination

[mattgle]

Adds a static ShieldNote.create factory so integrators can construct a shield note for a recipient from their already-parsed master public key, instead of only reconstructing notes from scanned on-chain data.

The factory takes masterPublicKey, a full pre-fee value, ERC20 or ERC721 tokenData, and an optional 16-byte random (generated via a browser-safe source when omitted). It derives the note public key through the existing Note.computeNotePublicKey path and validates token data with the shared assertValidNoteToken, so origination-side and scan-side commitment math and validation share one implementation. Inputs are validated up front: key and random lengths, supported token type, all-zero tokenSubID for ERC20, value of exactly 1 for ERC721, and a positive value within the contract's uint120 preimage bound. No fee math — the shield fee is deducted on-chain.

Tests include fixed vectors derived with @railgun-community/circomlibjs poseidon (and ethereum-cryptography keccak256 for the NFT token hash) asserting npk and both ERC20 and ERC721 commitment hashes match the reference implementation, msgpack serialize/deserialize round-trips, and per-rule validation rejections. deserialize now normalizes an absent blockNumber to undefined (msgpack decodes missing fields as null), matching the existing shieldFee handling.

Stacked on #28; retargets to dev once that merges.

[bhflm]

Looks good overall, but I think this one needs a small fix before merge.

in src/notes/index.ts if we're exporting ShieldNoteParams here, should we also export ShieldNoteCreateParams?otherwise the new factory params type isn't available from the barrel export, right?