feat(clients/js): add confidential mint/burn instruction plan helpers

Token-2022's /confidential subpath ships InstructionPlan helpers for
transfer, withdraw and configure, but nothing for confidential mint/burn
— downstream consumers had to re-derive the three-proof orchestration
themselves. Add helpers so that logic lives upstream.

- getConfidentialMintInstructionPlan / getConfidentialBurnInstructionPlan:
  build the equality, batched grouped-3-handles validity and U128 range
  proofs, verify them via context-state accounts, and assemble the token
  instruction — reusing the existing transfer/withdraw plumbing.
- getUpdateConfidentialMintBurnDecryptableSupplyInstructionFromSupply:
  re-encrypt the decryptable supply under the supply AES key (u64-guarded).
- addWithLoHiCiphertexts: the one missing arithmetic primitive, mirroring
  subtractWithLoHiCiphertexts, for homomorphically adding the minted
  amount to the encrypted supply.
- test setup: createConfidentialMintBurnMint, initializing a mint with
  both the ConfidentialTransferMint and ConfidentialMintBurn extensions.

The helpers reuse the primitives introduced by #1253 rather than adding
their own: getRequiredMintExtension for the ConfidentialMintBurn lookup
and assertU64Amount for the supply guards. getAuditorElGamalPubkey is
widened to a structural input so transfer, transfer-with-fee, mint and
burn all resolve the auditor key through one function, and main's
now-redundant getRequiredConfidentialTransferMintExtension folds into
the generic lookup.

Covered by a new offline arithmetic test and a validator e2e that
exercises mint -> apply -> burn -> apply-pending-burn -> update-supply
against the on-chain ZK verifiers.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: eldarik

clients/js/src/confidentialTransferArithmetic.ts

@@ -51,6 +51,15 @@ export function extractCiphertextFromGroupedBytes(groupedCiphertext: ReadonlyUin
     return ciphertext;
 }
 
+function addCiphertexts(left: ReadonlyUint8Array, right: ReadonlyUint8Array) {
+    const leftPoints = ciphertextToPoints(left);
+    const rightPoints = ciphertextToPoints(right);
+    return pointsToCiphertext(
+        leftPoints.commitment.add(rightPoints.commitment),
+        leftPoints.handle.add(rightPoints.handle),
+    );
+}
+
 function subtractCiphertexts(left: ReadonlyUint8Array, right: ReadonlyUint8Array) {
     const leftPoints = ciphertextToPoints(left);
     const rightPoints = ciphertextToPoints(right);
@@ -70,6 +79,20 @@ function combineLoHiCiphertexts(ciphertextLo: ReadonlyUint8Array, ciphertextHi:
     );
 }
 
+/**
+ * Combines lo/hi ciphertext halves (hi << bitLength + lo) and adds the result
+ * to `left`. Used to compute the new encrypted supply ciphertext after a
+ * confidential mint.
+ */
+export function addWithLoHiCiphertexts(
+    left: ReadonlyUint8Array,
+    ciphertextLo: ReadonlyUint8Array,
+    ciphertextHi: ReadonlyUint8Array,
+    bitLength: bigint,
+) {
+    return addCiphertexts(left, combineLoHiCiphertexts(ciphertextLo, ciphertextHi, bitLength));
+}
+
 /**
  * Combines lo/hi ciphertext halves (hi << bitLength + lo) and subtracts the
  * result from `left`. Used to compute the new available-balance ciphertext