Add musl/Alpine Linux support with static linking

- Add libc parameter to redis_stack() to support both glibc and musl builds
- musl builds use static linking with libc malloc (no jemalloc/TLS dependencies)
- Share download between glibc and musl variants to avoid duplication
- Add redis_stack_musl target for Alpine-compatible binaries
- Update README to document musl build requirements and remove manual extraction
- Document required dependencies: musl-tools, musl-dev

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Redis Stack v7.4.2

Initial commit