Echo v0.0.1-alpha.7

macOS CI: fix LLVM clang as CC breaking llvm-sys (missing SDK inttypes.h).

• System `CC` for build scripts
• LLVM tarball `clang` + `ld64.lld` only via `RUSTFLAGS` for Rust link
• Windows remains on the alpha.6 xml2s.lib path (already green)

Install

```bash
curl -fsSL https://raw.githubusercontent.com/modoterra/echo/main/scripts/install.sh \
| bash -s -- from-release v0.0.1-alpha.7
```