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/listing.ts

@@ -0,0 +1,54 @@
+import { address, type Account, type Address, type Rpc } from '@solana/kit'
+import { type GetProgramAccountsApi } from '@solana/rpc-api'
+import { type GetAccountInfoApi } from '@solana/rpc-api'
+import type { Base64EncodedBytes } from '@solana/rpc-types'
+import {
+  decodeSolistingStateListingAccount,
+  fetchSolistingStateListingAccount,
+  SOLISTING_STATE_LISTING_ACCOUNT_DISCRIMINATOR,
+  type SolistingStateListingAccount,
+} from './generated/solisting/src/generated/index.js'
+
+const PROGRAM_ADDRESS = address('DzwUAbpRvqcbA8QsEkRbZeXG4TEho5782cMySodrUHBU')
+
+export type ListingAccountWithPda = Account<SolistingStateListingAccount>
+export type GpaRpc = Rpc<GetProgramAccountsApi>
+export type GetRpc = Rpc<GetAccountInfoApi>
+
+export async function fetchAllListings(rpc: GpaRpc): Promise<ListingAccountWithPda[]> {
+  const disc = SOLISTING_STATE_LISTING_ACCOUNT_DISCRIMINATOR
+  const discBase64 = Buffer.from(disc).toString('base64') as Base64EncodedBytes
+  const results = await rpc
+    .getProgramAccounts(PROGRAM_ADDRESS, {
+      encoding: 'base64',
+      filters: [{ memcmp: { offset: 0n, bytes: discBase64, encoding: 'base64' } }],
+    })
+    .send()
+  return (results as Array<{ pubkey: Address; account: { executable: boolean; lamports: bigint; owner: Address; space: bigint; data: [string, 'base64'] } }>).map((r) => {
+    const data = new Uint8Array(Buffer.from(r.account.data[0], 'base64'))
+    return decodeSolistingStateListingAccount({
+      address: r.pubkey,
+      data,
+      executable: r.account.executable,
+      lamports: r.account.lamports as unknown as import('@solana/rpc-types').Lamports,
+      programAddress: r.account.owner,
+      space: r.account.space,
+      exists: true,
+    }) as ListingAccountWithPda
+  })
+}
+
+export async function fetchListingsBySeller(
+  rpc: GpaRpc,
+  seller: Address,
+): Promise<ListingAccountWithPda[]> {
+  const all = await fetchAllListings(rpc)
+  return all.filter((l) => l.data.seller === seller)
+}
+
+export async function fetchListing(
+  rpc: GetRpc,
+  addr: Address,
+): Promise<ListingAccountWithPda | null> {
+  return fetchSolistingStateListingAccount(rpc, addr).catch(() => null)
+}