[Contract] Fix USDC multi-hop ABI encoding in ZapPlotLinkV2

[realproject7]

Summary

USDC zap is currently disabled on the frontend (PR #466) because the multi-hop swap reverts. The root cause is in ZapPlotLinkV2._executeV4MultiHopSwapExactIn — the SWAP_EXACT_IN action params are ABI-encoded differently from what the Universal Router expects.

ETH single-hop and HUNT (MCV2 bonding curve) routes work fine. Only the USDC→ETH→PLOT multi-hop path is broken.

Root Cause (from #464 investigation)

The USDC→ETH→PLOT multi-hop swap reverts inside Universal Router's unlockCallback with empty revert data (1618 gas consumed). The SWAP_EXACT_IN action params encoding in _executeV4MultiHopSwapExactIn doesn't match what the deployed Router expects.

Fix approach

1. Compare with MintPad's ZapUniV4MCV2 (0xa2e7BcA51A84Ed635909a8E845d5f66602742A75) — find successful USDC multi-hop txs on Basescan and compare calldata encoding
2. Check if the USDC/ETH V4 pool exists with the configured params (fee=500, tickSpacing=10) on Base mainnet
3. Verify PathKey encoding order matches Universal Router V4 spec for multi-hop swaps
4. Fix _executeV4MultiHopSwapExactIn and _executeV4MultiHopSwapExactOut encoding
5. Redeploy ZapPlotLinkV2 to Base mainnet, verify on Sourcify

After contract fix

1. Update ZAP_PLOTLINK in lib/contracts/constants.ts with new address
2. Re-enable USDC in SUPPORTED_ZAP_TOKENS (was removed in PR Disable USDC zap pending contract fix (#464) #466)
3. Execute a successful USDC zap trade on mainnet and document tx hash

Contract references

• Current ZapPlotLinkV2: 0x04f557F8D2806B34FC832a534c08DF514D4dfEeF
• MintPad ZapUniV4MCV2: 0xa2e7BcA51A84Ed635909a8E845d5f66602742A75
• Multi-hop code: plotlink-contracts/src/ZapPlotLinkV2.sol lines 463-535
• USDC: 0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913

Acceptance Criteria

• Multi-hop ABI encoding fixed
• Contract redeployed on Base mainnet
• Verified on Sourcify + Basescan
• ZAP_PLOTLINK updated in plotlink constants
• USDC re-added to SUPPORTED_ZAP_TOKENS
• Successful USDC zap trade tx hash documented in PR
• Build passes

Branch

task/466-usdc-multihop-fix in plotlink-contracts repo (+ plotlink for constants + re-enable USDC)