Starknet: Implement Cairo HTLC Contract

[0xdevcollins]

Severity: MEDIUM — Starknet Chain Support Is Non-Functional

Problem

contract/starknet/src/htlc.cairo is completely empty (0 bytes). The bridge router routes Starknet payments via Layerswap, which handles bridging, but the on-chain HTLC on Starknet side for locking payer funds does not exist.

Implementation Required

```cairo
use starknet::ContractAddress;
use starknet::get_caller_address;
use starknet::get_block_timestamp;
use openzeppelin::token::erc20::interface::{IERC20Dispatcher, IERC20DispatcherTrait};

#[derive(Drop, Serde, starknet::Store)]
struct LockEntry {
sender: ContractAddress,
receiver: ContractAddress,
token: ContractAddress,
amount: u256,
hashlock: felt252, // sha256 of secret
timelock: u64,
withdrawn: bool,
refunded: bool,
}

#[starknet::contract]
mod HTLCStarknet {
use super::*;
use starknet::storage::{Map, StorageMapReadAccess, StorageMapWriteAccess};

#[storage]
struct Storage {
    locks: Map<felt252, LockEntry>,  // lockId => LockEntry
    paused: bool,
    admin: ContractAddress,
}

#[event]
#[derive(Drop, starknet::Event)]
enum Event {
    Locked: LockedEvent,
    Withdrawn: WithdrawnEvent,
    Refunded: RefundedEvent,
}

#[abi(embed_v0)]
impl HTLCImpl of super::IHTLC<ContractState> {
    fn lock(ref self, receiver, token, amount, hashlock, timelock) -> felt252
    fn withdraw(ref self, lock_id: felt252, preimage: felt252) -> bool
    fn refund(ref self, lock_id: felt252) -> bool
    fn get_lock(self: @ContractState, lock_id: felt252) -> LockEntry
    fn pause(ref self)
    fn unpause(ref self)
}

}
```

Also Required

• Scarb.toml build config
• Deploy script using starkli
• Tests using snforge (Starknet Foundry)
• Add STARKNET_HTLC_ADDRESS and STARKNET_RELAY_PRIVATE_KEY to .env.example

Note on Layerswap Integration

For the beta, Layerswap handles Starknet bridging at the protocol level. The Cairo HTLC is needed for a fully non-custodial Starknet integration in the future. This can be deprioritized if beta scope excludes Starknet.

Acceptance Criteria

• Cairo HTLC contract compiles with scarb build
• lock(), withdraw(), refund() match EVM/Soroban semantics
• Emergency pause implemented
• Tests with snforge: min 8 test cases
• Deploy script using starkli
• Deployed to Starknet testnet (Sepolia)