chore: add CLAUDE.md for code quality conventions

CLAUDE.md

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+# Trevm
+
+## Commands
+
+- `cargo +nightly fmt` - format
+- `cargo clippy --all-features --all-targets` - lint with features
+- `cargo clippy --no-default-features --all-targets` - lint without
+- `cargo t --all-features` - test with all features
+- `cargo t --no-default-features` - test without features
+
+Pre-commit: clippy (both feature sets) + fmt. Never use `cargo check/build`.
+
+## Style
+
+- Functional combinators over imperative control flow
+- `let else` for early returns, avoid nesting
+- No glob imports; group imports from same crate; no blank lines between imports
+- Private by default, `pub(crate)` for internal, `pub` for API only; never `pub(super)`
+- `thiserror` for library errors, never `anyhow` or `eyre` in library code
+- `tracing` for instrumentation: instrument work items not long-lived tasks; `skip(self)` on methods
+- Builders for structs with >4 fields or multiple same-type fields
+- Tests: fail fast with `unwrap()`, never return `Result`; unit tests in `mod tests`
+- Rustdoc on all public items with usage examples; hide scaffolding with `#`
+- `// SAFETY:` comments on all unsafe blocks
+- Minimize generics in user-facing API; provide concrete types where possible
+
+## Crate Notes
+
+- Single library crate using the **typestate pattern** to enforce correct EVM usage at compile time
+- State flow: `EvmNeedsCfg` -> `EvmNeedsBlock` -> `EvmNeedsTx` -> `EvmReady` -> `EvmTransacted`
+- Extensive feature flags: test with both `--all-features` and `--no-default-features`
+- Key features: `call`, `concurrent-db`, `estimate_gas`, `tracing-inspectors`, `alloy-db`, `test-utils`
+- Uses `#[cfg_attr(docsrs, doc(cfg(...)))]` for feature-gated documentation