feat! migrate @solana/pay to @solana/kit

[amilz]

Summary

• Breaking: migrates core library from @solana/web3.js to @solana/kit — all RPC calls, transaction building, and type signatures updated accordingly
• Introduces createSolanaPayClient convenience factory with merchant and wallet role plugins for a composable API
• Refactors transfer/verification internals: batched RPC calls, unified transfer types, improved reference normalization
• Add tests
• Updates CI to pnpm, adds lint/typecheck jobs; cleans up legacy examples

Note: I moved/removed some sample code, so lots of files changed but bulk of PR is ~10 files in core/src and relevant tests in core/test.

Sample

// Merchant — encode a payment URL, generate QR, find & validate
const merchant = createMerchantClient({
  rpcUrl: 'https://api.mainnet-beta.solana.com',
});

const url = merchant.pay.encodeURL({ recipient, amount });
const qr = merchant.pay.createQR(url);

const found = await merchant.pay.findReference(reference);
await merchant.pay.validateTransfer(found.signature, { recipient, amount });

// Wallet — parse a URL, build transfer instructions
const wallet = createWalletClient({
  rpcUrl: 'https://api.mainnet-beta.solana.com',
  payer: signer,
});

const parsed = wallet.pay.parseURL(url);
const instructions = await wallet.pay.createTransfer({ recipient, amount });

Breaking Changes

@solana/web3.js peer dependency is replaced by @solana/kit. Public API types (addresses, transactions, signers) now use Kit primitives. Consumers must migrate to @solana/kit types.

See core/README.md for updated usage.

Test Plan

make install
make test

Replaces #272 and #283
Ref #286

[lgalabru]

ship it 🚀

[lgalabru]

Thanks @amilz!