feat: add @solisting/sdk helpers and unit tests

Add hand-written SDK helper files (pda.ts, listing.ts, order.ts) with
GPA-based account fetching, discriminator filters, PDA derivation helpers,
and escrow ID derivation. Add pda.test.ts with 4 unit tests (all passing).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

sdk/src/pda.ts

@@ -0,0 +1,30 @@
+import { address, getAddressEncoder, type Address, type ProgramDerivedAddress } from '@solana/kit'
+import { findListingAccountPda, findOrderAccountPda } from './generated/solisting/src/generated/index.js'
+
+const PROGRAM_ADDRESS = address('DzwUAbpRvqcbA8QsEkRbZeXG4TEho5782cMySodrUHBU')
+
+export async function findListingPda(
+  seller: Address,
+  listingId: bigint,
+): Promise<ProgramDerivedAddress> {
+  return findListingAccountPda({ seller, listingId }, { programAddress: PROGRAM_ADDRESS })
+}
+
+export async function findOrderPda(
+  listingAccount: Address,
+  buyer: Address,
+  orderId: bigint,
+): Promise<ProgramDerivedAddress> {
+  return findOrderAccountPda({ listingAccount, buyer, orderId }, { programAddress: PROGRAM_ADDRESS })
+}
+
+/**
+ * Replicates the program's escrow_id derivation:
+ *   u64::from_le_bytes(order_pda.to_bytes()[0..8])
+ * Must match exactly — used when building create_order instructions.
+ */
+export function deriveEscrowId(orderPda: Address): bigint {
+  const bytes = getAddressEncoder().encode(orderPda) // 32-byte Uint8Array
+  const view = new DataView(bytes.buffer, bytes.byteOffset, bytes.byteLength)
+  return view.getBigUint64(0, true) // little-endian, first 8 bytes
+}