contracts: migrate contract to submodule

[hyunsooda]

Proposed changes

Add kaia-system-contracts as git submodule — single source of truth for system contract .sol files and compiled artifacts

• Add submodule: contracts/kaia-system-contracts/ → kaia-system-contracts pointing to main
• Add --artifact mode to abigenw: generate Go bindings directly from pre-compiled Hardhat/Forge JSON artifacts instead of recompiling .sol from source. Permissionless contracts (v3.0) use Forge artifacts from the submodule.
• Consolidate all Go bindings under contracts/bindings/: move generated .go files out of testing/, libs/, service_chain/, contracts/system_contracts/* into bindings/. All callers updated.
• Flatten contracts/contracts/ → contracts/: remove the redundant nesting. hardhat.config.ts updated to use TASK_COMPILE_SOLIDITY_GET_SOURCE_PATHS subtask with explicit glob instead of paths.sources.
• Activate go:generate directives: all non-WARNING // //go:generate lines uncommented. WARNING-guarded bindings (credit.go, SimpleBlsRegistry.go, Registry.go, proxyv4/Proxy.go) remain commented — BinRuntime must match mainnet-deployed bytecode.
• Rewrite contracts/README.md for the new directory structure.

Types of changes

• 🐛 Bug fix
• ✨ Non-hardfork changes (node upgrade not required)
• 💥 Hardfork / consensus-breaking changes
• 🧪 Test improvements
• 🧰 CI / build tool
• ♻️ Chore / Refactor / Non-functional changes

Checklist

• 📖 I have read the CONTRIBUTING GUIDELINES doc
• 📝 I have signed in the PR comment I have read the CLA Document and I hereby sign the CLA in first time contribute after having read CLA
• 🟢 Lint and unit tests pass locally with my changes ($ make test)

Related issues

Further comments

kaia-system-contracts submodule serves as the single source of truth. Going forward, system contract changes are managed by updating the submodule and running go generate to regenerate bindings.

bindings/multicall/ uses MultiCallContract from contracts-v3.0. No separate v2.2 binding is generated because the v3.0 MultiCallContract subsumes v2.2's MultiCall. It is pinned to solc 0.8.19 so its bytecode targets pre-Shanghai EVM (no PUSH0) — required since MultiCallContract is injected into state on every block.

hardhat.config.ts uses a TASK_COMPILE_SOLIDITY_GET_SOURCE_PATHS subtask override instead of paths.sources = "." because setting paths.sources = "." causes Hardhat to treat node_modules/ as a local source directory, breaking OpenZeppelin imports with HH1006.

Directory structure after this PR:

contracts/
├── abigenw                    # Go binding generator script
├── generate.go                # go generate directives
├── bindings/                  # Generated Go bindings (do not edit manually)
│   ├── abv2data/
│   ├── addressbookv2/
│   ├── auction/
│   ├── beacon/
│   ├── bridge/
│   ├── cnstakingv4/
│   ├── cnstakingv4factory/
│   ├── gov/
│   ├── kip113/
│   ├── kip149/
│   ├── kip247/
│   ├── kip13/
│   ├── misc/
│   ├── multicall/
│   ├── proxyv4/
│   ├── proxyv5/
│   ├── publicdelegation/
│   ├── rebalance/
│   ├── testing/               # Test-only bindings
│   └── uniswap/
├── kaia-system-contracts/     # Git submodule — upstream .sol + artifacts
├── libs/                      # Vendored Solidity libraries (.sol only)
├── service_chain/             # Service chain bridge contracts (.sol only)
└── testing/                   # Mock and testing contracts (.sol only)

TODO

• kaia-system-contracts submodule currently points to a private repository. Switch to the public repo URL once it is released.
• .gitmodules currently pins branch = main. Replace with the release tag once ksc cuts one so the submodule points to a stable, immutable ref instead of a moving branch head.