Add the initial CLAUDE.md file

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
+
+PolkaVM is a general-purpose RISC-V based virtual machine targeting RV32EM. It provides secure sandboxed execution with both an interpreter and a JIT compiler backend.
+
+## Build Commands
+
+```bash
+# Build all workspace crates
+cargo build
+
+# Build with release optimizations
+cargo build --release
+
+# Run all tests (both dev and release)
+./ci/jobs/build-and-test.sh
+
+# Run tests for a specific crate
+cargo test -p polkavm
+cargo test -p polkavm-linker
+cargo test -p polkavm-common
+
+# Run tests with all features
+cargo test -p polkavm-common --all-features
+
+# Run a single test
+cargo test -p polkavm -- test_name
+
+# Linting
+./ci/jobs/clippy.sh
+cargo clippy -p polkavm
+
+# Formatting
+cargo fmt --check --all
+cargo fmt --all
+```
+
+## Guest Programs
+
+Guest programs (programs that run inside the VM) are in `guest-programs/` and use a separate workspace with nightly Rust:
+
+```bash
+cd guest-programs
+
+# Build guest programs (uses custom RISC-V target)
+cargo build --release
+
+# The custom target is at: crates/polkavm-linker/targets/legacy/riscv32emac-unknown-none-polkavm.json
+```
+
+Guest programs require nightly Rust (`nightly-2025-05-10`) with `rust-src` component for `-Z build-std`.
+
+## Architecture
+
+### Core Crates (`crates/`)
+
+- **polkavm**: Main VM crate with `Engine`, `Module`, `Instance` API. Contains:
+  - `interpreter.rs`: Cross-platform interpreter backend
+  - `compiler.rs`: JIT compiler (x86_64 Linux/macOS/FreeBSD)
+  - `sandbox/`: Process isolation for compiled code
+  - `api.rs`: Public API types
+  - `linker.rs`: Runtime linking of host functions
+
+- **polkavm-common**: Shared types and utilities
+  - `program.rs`: Program blob format, instruction definitions, parsing
+  - `assembler.rs`: Assembly/disassembly support
+  - `abi.rs`: VM ABI definitions (memory map, calling conventions)
+
+- **polkavm-linker**: Offline linker that transforms ELF to PolkaVM format
+  - Reads RISC-V ELF files from guest compilation
+  - Performs register allocation via regalloc2
+  - Outputs `.polkavm` program blobs
+
+- **polkavm-zygote**: Linux-only process template for sandboxed execution (separate workspace)
+
+- **polkavm-derive**: Proc macros for guest programs (`#[polkavm_import]`, `#[polkavm_export]`)
+
+### Tools (`tools/`)
+
+- **polkatool**: CLI for inspecting/disassembling `.polkavm` blobs
+- **spectool**: Generates test vectors for VM specification
+- **gastool**: Gas metering analysis (Linux only)
+- **benchtool**: Performance benchmarking suite
+
+### Execution Backends
+
+The VM supports two backends controlled via `Config::set_backend()` or `POLKAVM_BACKEND` env var:
+- `interpreter`: Cross-platform, always available
+- `compiler`: JIT to native code, requires x86_64 + (Linux | macOS with `generic-sandbox` feature | FreeBSD with `generic-sandbox` feature)
+
+### Environment Variables
+
+- `POLKAVM_BACKEND`: `auto`, `compiler`, `interpreter`
+- `POLKAVM_SANDBOX`: `auto`, `linux`, `generic`
+- `POLKAVM_ALLOW_EXPERIMENTAL`: Enable experimental features
+- `POLKAVM_TRACE_EXECUTION`: Enable execution tracing (for debugging)
+
+## Testing
+
+```bash
+# Full test suite
+./ci/jobs/build-and-test.sh
+
+# Run with execution tracing
+POLKAVM_TRACE_EXECUTION=1 POLKAVM_ALLOW_INSECURE=1 cargo run -p hello-world-host
+
+# Fuzzing (requires cargo-fuzz)
+cd fuzz
+cargo fuzz run fuzz_polkavm -- -runs=10000
+```
+
+## Rust Version
+
+- Main workspace: Rust 1.86.0 (stable)
+- Guest programs: nightly-2025-05-10
+
+## Key Types
+
+- `Engine`: VM engine instance, configured via `Config`
+- `Module`: Compiled program, created from `ProgramBlob`
+- `Instance`: Execution instance with memory state
+- `Linker`: Links host functions to module imports
+- `ProgramBlob`: Serialized program format