🤖 Add CLAUDE.md

Author: zeenix

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.
+
+## Commands
+
+Build and test:
+```bash
+cargo build
+cargo test --features serde
+cargo test --no-default-features --features heapless,serde
+```
+
+Linting and formatting:
+```bash
+cargo +nightly fmt
+cargo clippy
+cargo doc --all-features
+```
+
+## Architecture
+
+This crate provides abstractions over `alloc` and `heapless` collections,
+allowing code to work with both heap-allocated and stack-allocated data
+structures through a unified API.
+
+### Key Design Patterns
+
+- **Feature-based compilation**: Either `alloc` (default) or `heapless` feature
+  must be enabled, but not both simultaneously
+- **Capacity parameterization**: All types use const generic `N` parameter for
+  capacity control - initial capacity for `alloc`, maximum capacity for
+  `heapless`
+- **Fallible operations**: Methods return `Result<T, Error>` to handle capacity
+  limits in `heapless` mode, though they're infallible with `alloc`
+- **Backend abstraction**: Internal `Inner<T, N>` type aliases provide
+  conditional compilation between `alloc::vec::Vec<T>` and `heapless::Vec<T, N>`
+
+### Core Modules
+
+- `vec.rs`: Vec implementation with unified API over both backends
+- `string.rs`: String implementation with unified API over both backends  
+- `error.rs`: Error types (`BufferOverflow`, `Utf8Error`) and Result alias
+- `lib.rs`: Feature gates, re-exports, and serde integration tests
+
+The crate uses `#![no_std]` and requires explicit `extern crate alloc` when
+alloc feature is enabled.