feat(ton): add TON network support (native + jetton)

[web3luka]

Description

This PR adds TON (The Open Network) support to x402.
It introduces a new network adapter that enables merchants and facilitators to accept payments in TON native coins as well as Jettons (TEP-74 standard).

Key changes:

• [x] Implemented verifyTONExact for native and jetton payments
• [x] Added address normalization across raw/bounceable / non-bounceable formats
• [x] Added memo validation (x402: strict mode) with fallback for legacy memos
• [x] Unified error codes for better integration with existing adapters
• [x] Utility helpers: normalizeTonAddress, getTonExplorerUrl, validateMemo, toAtomic
• [x] Unit tests covering native + jetton flows, decimal mismatch, forward_ton_amount edge cases, cross-format addresses
• [x] Explorer integration via tonviewer.com (mainnet + testnet)

---

Tests

• [x] Added src/ton/index.test.ts with >180 cases:
  • Native transfers with memo
  • Jetton transfers with forward payload (memo)
  • Decimal mismatch/amount mismatch
  • Cross-format address normalization
  • Invalid memo and expired invoice handling
• [x] Verified unit tests with pnpm test inside /typescript
• [x] Manually checked explorer URL generation for mainnet and testnet

---

Checklist

• [x] I have formatted and linted my code (pnpm format && pnpm lint)
• [x] All new and existing tests pass (pnpm test)
  
• [x] My commits are signed (verified via git log --show-signature)

[cb-heimdall]

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