feat: add mutable protocol-level multisig account support

[digitalSloth]

• src/model/primitives/address.ts, index.ts, src/index.ts: add Address.multisigByte/isMultisig/fromMultisigCreation and the MULTISIG_ADDRESS vanity constant.
• src/model/nom/accountBlock.ts: add optional multisigAuth field to AccountBlockTemplate/AccountBlock with base64 JSON round-trip; omitted entirely from toJson when unset.
• src/utilities/block.ts: comment-only note in getTxHash documenting that multisigAuth is excluded from the consensus preimage (INV-1); extract validateReceiveBlock out of checkAndSetFields (behavior-preserving, INV-2); add new exported freezeBlock/signBlock/assembleMultisigAuth primitives for both send and receive multisig blocks.
• src/embedded/multisig.ts, src/model/embedded/multisig.ts, src/api/embedded/multisig.ts: new 12th embedded contract (CreateMultisig/ ChangePolicy ABI, MultisigPolicyInfo/MultisigRecordInfo models, MultisigApi with getPolicy/createMultisig/changePolicy), wired into src/embedded/index.ts, src/model/embedded/index.ts, src/api/embedded/embedded.ts and src/index.ts (MultisigContract only, matching the htlc precedent — MultisigApi is not surfaced top-level).
• src/client/nodeErrors.ts (new) + src/client/index.ts: central typed node-error registry (mapNodeError) and a flat Znn...Exception hierarchy rooted at ZnnEmbeddedContractException extends ZnnClientException.
• src/client/http.ts, src/client/websocket.ts, src/api/ledger.ts: route both RPC-error channels and the publishRawTransaction non-null-result channel through mapNodeError.
• test/api/ledger.spec.ts: update the publish-error assertion to match the new typed-error shape (was asserting the old NETWORK_ERROR message).

[mrstockholmz]

Reviewed at f1b6e0f. Clean SDK counterpart to the go-zenon#72 multisig work.

The three-phase pipeline (freeze → sign → assemble) is the right shape. Freezing before collecting signatures means every signer signs the identical hash, including across the toJson/fromJson hand-off. The signBlock guard against unfrozen blocks catches the most likely integration mistake.

The hash-invariance tests are the important ones. They pin that multisigAuth (unset, set, mutated, or round-tripped) never changes getTxHash, which matches the node excluding the auth data from the block hash. Omitting multisigAuth from toJson when it is unset keeps existing consumers unaffected.

One doc mismatch with the node side: in go-zenon#72, VerifyThresholdSignatures requires exactly Threshold signatures (len(signatures) != int(policy.Threshold) returns false). The order-independence claim is correct, since the verifier trial-matches against distinct signers. But docs/multisig.md step 4 says "Assemble and publish once >= threshold signatures are collected". A 2-of-3 wallet that collects all three and passes them to assembleMultisigAuth would be rejected with MultisigThresholdMismatchException. Suggest changing the docs to "exactly threshold". Optionally, assembleMultisigAuth could take the threshold, or the doc comment could say that the caller must pass exactly that many signatures.

— mr.stockholm.Z