docs: initial CLAUDE.md

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with
+code in this repository.
+
+## Project Overview
+
+`rain-erc` is a pure-Rust crate of ERC-related utilities for the Rain Protocol
+ecosystem. Currently contains one module:
+
+- `erc165` — probe arbitrary contracts for `IERC165` support against any
+  `alloy::providers::Provider`. Spec-compliant: execution reverts on the probe
+  count as "interface not supported" per
+  [EIP-165](https://eips.ethereum.org/EIPS/eip-165); only RPC / decoding errors
+  are returned as `Err`.
+
+No Solidity, no submodules, no path-deps. Everything else (alloy, thiserror) is
+on crates.io.
+
+## Build Environment
+
+Nix flake with the slim `rust-shell` as the default devShell. Enter with
+`nix develop` or prefix commands with `nix develop -c`.
+
+| Task         | Command                                                                                  |
+| ------------ | ---------------------------------------------------------------------------------------- |
+| Native tests | `nix develop -c cargo test`                                                              |
+| WASM build   | `nix develop -c cargo build --target wasm32-unknown-unknown`                             |
+| Format check | `nix develop -c cargo fmt --all -- --check`                                              |
+| Clippy       | `nix develop -c cargo clippy --all-targets --all-features -- -D warnings -D clippy::all` |
+
+Run a single test:
+
+```sh
+nix develop -c cargo test test_name
+```
+
+## Architecture
+
+### `src/erc165/mod.rs`
+
+- `supports_erc165<P: Provider>(provider, addr) -> Result<bool, Erc165Error>` —
+  orchestrates two probes per EIP-165:
+  - `check1`: `supportsInterface(0x01ffc9a7)` must return `true`.
+  - `check2`: `supportsInterface(0xffffffff)` must return `false` (or revert,
+    which counts as `false`).
+- `Erc165Error` distinguishes transport/decoding errors from
+  interface-unsupported results. Revert errors are classified by inspecting the
+  alloy `ContractError`.
+
+Tests use `alloy::providers::mock::Asserter` driven by
+`ProviderBuilder::new().connect_mocked_client(asserter)`. No real network calls;
+no submodules.
+
+## CI
+
+`.github/workflows/rainix-rs.yaml` is a thin caller of
+`rainlanguage/rainix/.github/workflows/rainix-rs.yaml@main`. The upstream
+reusable workflow covers rs-static + rs-test (ubuntu/macos) + rs-wasm +
+rs-wasm-test.
+
+`.github/workflows/manual-rs-release.yml` is a manual `workflow_dispatch` that
+runs `cargo release LEVEL` and publishes to crates.io. Triggered by maintainers
+when a release is ready; there's no auto-publish on push to main.
+
+## Release
+
+1. Bump `version` in `Cargo.toml` on a PR.
+2. After merge, sync `Cargo.lock` (cargo update -p rain-erc) on a second PR —
+   required because `cargo release` refuses to run with a dirty tree.
+3. Once main is at the new version with a clean lockfile, trigger the `Release`
+   workflow with the appropriate version-level (`release` to drop a pre-release
+   suffix, `patch`/`minor`/`major` to bump).
+
+## License
+
+CAL-1.0. SPDX headers per REUSE 3.2.