feat(contract): HTLC atomic swaps with key-bound secret hash

[success-OG]

[Contract] Hashed Timelock Contract (HTLC) for Atomic Swaps

closes #261

Summary

Implements a full HTLC in Soroban for cross-chain atomic swaps with a cryptographic
defense against intercept attacks by the agent or any third party.

What's included

• New htlc contract added to the workspace
• init_swap — locks tokens into the contract, stores swap keyed by a derived swap_id.
  Caller provides secret_hash = SHA-256(preimage || recipient_pubkey), binding the
  hash to the intended recipient's Stellar public key at creation time.
• claim — recipient reveals (preimage, recipient_pubkey). Contract recomputes
  SHA-256(preimage || recipient_pubkey) and checks it matches the stored hash.
  An attacker who intercepts the preimage cannot claim because their pubkey won't
  produce the correct hash.
• refund — initiator reclaims funds after expiry_ledger has passed.
• get_swap — read-only view of any swap by ID.
• 8 unit tests: happy-path init, successful claim, wrong preimage rejected,
  intercept attack blocked, refund after expiry, refund before expiry blocked,
  double-claim blocked, expired claim blocked.

Security note

The key-binding SHA-256(preimage || recipient_pubkey) means even if the preimage
leaks on-chain or off-chain, only the address whose pubkey was committed at swap
creation can successfully call claim.

Branch

feat/htlc-atomic-swaps

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