feat: INativeDoge token-duality interface + Foundry test shim

[dghelm]

Summary

Groundwork for using Celo-style token duality (an ERC20-shaped token whose balances are native account balances) in DogeOS contracts — specifically so the upcoming payment/intent-ledger work can be developed and tested against duality semantics now, before the protocol pieces exist.

True duality cannot be implemented in pure EVM: moving another account's native balance requires a protocol-level native-transfer precompile (Celo's CELO/GoldToken model). Enabling it for real on DogeOS needs, tracked separately:

1. a native-transfer precompile in l2geth/revm (callable only by the duality token contract), with prover/stateless-verifier support;
2. a predeploy token contract at a canonical DogeOSPredeploy address calling that precompile;
3. genesis integration.

What this PR ships

• src/dogeos/INativeDoge.sol — the interface contracts should code against, with production semantics specified: balanceOf(a) == a.balance (no storage balances), transfers move native value without executing recipient code (no receive()/fallback() hooks — matching Celo's precompile behavior), storage allowances.
• src/test/mocks/DualityDogeShim.sol — TEST-ONLY functional shim implementing the interface in Foundry via vm.deal (the in-test stand-in for the future precompile). Semantically faithful, including the no-recipient-hooks property; explicitly not valid for gas measurement. Placed under src/test/mocks/ so hardhat never compiles it (it imports forge-std) and slither/coverage exclude it.
• src/test/dogeos/DualityDogeShim.t.sol — 11 tests: native-balance mirroring, transfer/transferFrom/allowance semantics (incl. infinite-allowance no-decrement), insufficient balance/allowance reverts with exact errors, reverting-receive() cannot block a transfer, conservation fuzz.

Test plan

• forge test --evm-version cancun — 432 tests pass (11 new)
• npx hardhat compile — clean (interface compiles; shim correctly excluded)

Relation to other work

• PR feat: DogeSig library + DogeP2PKHVerifier predeploy for Dogecoin signmessage verification #39 (DogeSig/DogeP2PKHVerifier) is independent; the payment-contract branch (experiment/payment-contract) will build on both.
• Gas comparisons for duality-style transfers live in PR feat: DogeSig library + DogeP2PKHVerifier predeploy for Dogecoin signmessage verification #39's benchmark (DualityDoge value-call emulation) — that mock answers "what does it cost," this shim answers "does my contract behave correctly against it."

🤖 Generated with Claude Code

[dghelm]

Superseded by #51