v2026.4.18

Fixed

• Docker image builds on linux/arm64 again. The builder stage now installs gcc and libc6-dev so Python extensions that lack a prebuilt aarch64 wheel (notably fastavro, pulled in transitively via pydantic-ai-slim[cohere] → cohere) compile from sdist instead of failing with gcc: No such file or directory. No code change; PyPI wheel is byte-identical to 2026.4.17.