Add CLAUDE.md with project context for Claude Code

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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
+
+SDLamp2 is a simple SDL2-based audio player written in C, designed for a child to use. It plays long m4a/mp3 files (fairy tale cassette rips) with a cassette-player-like UI: rewind, stop, play, fast-forward, next tape. Inspired by Winamp.
+
+See `docs/sdlamp2-fsd.md` for the full functional specification and changelog.
+
+## Build and Run
+
+**Dependencies:** SDL2, SDL2_image, FFmpeg libraries (libavformat, libavcodec, libavutil, libswresample). Installed via system package manager; resolved at build time with `sdl2-config` and `pkgconf`.
+
+```sh
+./build.sh              # macOS — builds to build/sdlamp2
+./build_aarch64.sh      # Linux aarch64 variant
+cd build && ./sdlamp2 [audio_directory]
+```
+
+The binary must be run from `build/` because it loads `controls.png` from the current working directory. Audio directory defaults to cwd if not specified.
+
+No test suite, no linter, no Makefile/CMake.
+
+## Architecture
+
+Single-file C program: `src/sdlamp2.c` (~650 lines). No headers, no separate compilation units.
+
+Key sections in order:
+- **Decoder struct** — holds all FFmpeg state (format/codec contexts, swr resampler, album art texture)
+- **Position persistence** — `save_position()`/`load_position()` read/write a tab-separated `positions.txt` in the audio directory
+- **File scanning** — `scan_audio_files()` finds supported files (.m4a, .mp3, .wav, .ogg) in a directory
+- **Decoder lifecycle** — `decoder_open()` sets up FFmpeg + swr pipeline (planar float → interleaved stereo 48kHz), `decoder_pump()` feeds decoded audio into an SDL_AudioStream, `decoder_seek()` handles seeking
+- **File switching** — `switch_file()` saves position, opens new file, restores saved position
+- **Main loop** — event handling (mouse clicks on button rects), decoder pumping, SDL audio queue draining, EOF handling, rendering (album art, progress bar, control buttons from sprite sheet)
+
+Audio pipeline: FFmpeg decoder → libswresample (format conversion) → SDL_AudioStream (FIFO) → SDL audio device queue (push model, no callback).
+
+Uses SDL2 (not SDL3). Uses `#if LIBAVUTIL_VERSION_INT` preprocessor checks to support both old (channel_layout) and new (ch_layout) FFmpeg channel layout APIs.
+
+## Conventions
+
+- C formatted with Google style, `ColumnLimit: 100`
+- Keep it simple — Casey Muratori's semantic compression; don't refactor too soon
+- Minimize dependencies; discuss with owner before adding any
+- Non-fatal errors go to stderr and continue; fatal errors (SDL init failures) abort via `panic_and_abort()`
+- Update the changelog in `docs/sdlamp2-fsd.md` when making changes