Add CLAUDE.md (#449)

The initial version of the `CLAUDE.md` file.

Signed-off-by: xermicus <cyrill@parity.io>

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
+
+Revive is a Solidity compiler targeting RISC-V on PolkaVM. It uses `solc` (Ethereum Solidity compiler) as the frontend and LLVM as the backend. The main compiler binary is `resolc`.
+
+**Compilation pipeline:**
+1. `solc` lowers Solidity source to Yul IR
+2. `revive` lowers Yul IR to LLVM IR
+3. LLVM optimizes and emits RISC-V ELF via LLD
+4. PolkaVM linker creates the final PolkaVM blob
+
+## Build Commands
+
+**Prerequisites:** Requires `LLVM_SYS_181_PREFIX` environment variable pointing to a compatible LLVM 18.1.8 build. Download from releases or build with `make install-llvm`.
+
+```bash
+# Install resolc binary
+make install-bin
+
+# Run all quality checks (format, clippy, machete, tests)
+make test
+
+# Individual test targets
+make test-workspace      # All workspace tests (excludes llvm-builder)
+make test-integration    # Integration tests (requires resolc + geth evm tool)
+make test-resolc         # resolc package tests
+make test-yul            # Yul parser tests
+
+# Linting
+make format              # Check formatting (cargo fmt --all --check)
+make clippy              # Run clippy with --deny warnings
+
+# Run single test
+cargo test --package <crate-name> <test_name>
+cargo test --package revive-integration <test_name>
+```
+
+## Crate Architecture
+
+All crates live in `crates/`. Key crates:
+
+- **resolc** - Compiler driver binary and library; orchestrates the full pipeline
+- **revive-yul** - Yul lexer, parser, and LLVM IR lowering; implements visitor pattern for AST traversal
+- **revive-llvm-context** - LLVM code generation logic (decoupled from parser)
+- **revive-linker** - Links RISC-V ELF to PolkaVM blob via LLD and polkavm-linker
+- **revive-runner** - Executes contracts in simulated pallet-revive runtime; provides declarative test spec format
+- **revive-integration** - Integration test cases using revive-runner
+- **revive-differential** - Differential testing utilities against EVM
+- **revive-runtime-api** - Low-level runtime API bindings to pallet-revive
+- **revive-stdlib** - Compiler standard library components
+- **revive-common** - Shared constants and utilities
+- **lld-sys** - FFI bindings to LLVM's LLD linker
+
+## Testing Strategy
+
+Integration tests use **differential testing** against the Ethereum `solc`/EVM stack. Test specs are declarative JSON that define contract actions:
+
+```json
+{
+    "differential": true,
+    "actions": [
+        {"Instantiate": {"code": {"Solidity": {"contract": "ContractName"}}}},
+        {"Call": {"dest": {"Instantiated": 0}, "data": "..."}}
+    ]
+}
+```
+
+When `differential: true`, actions run on both EVM and PVM, asserting identical state changes.
+
+**Running integration tests requires** the `evm` tool from go-ethereum in `$PATH`.
+
+## Code Style Requirements
+
+- Use `BTreeMap` instead of `HashMap` for deterministic iteration (reproducible builds)
+- Avoid magic numbers/strings; use module constants
+- Avoid abbreviated names; use meaningful, readable symbols
+- No unnecessary macros
+- Avoid import aliasing; use parent/qualified paths for conflicts
+- Public items must have doc comments
+- Comments should provide semantic meaning, not restate code
+
+## Dependencies
+
+- Add as workspace dependencies in root `Cargo.toml`
+- Avoid pinning when possible
+- Always include `Cargo.lock` in PRs
+- Don't run `cargo update` with other changes (separate PR)
+
+## Rust Version
+
+Minimum supported: 1.85.0 (specified in workspace)