feat: pin zig 0.15.2 under /opt for the Emacs ghostel terminal

Arch's rolling repo ships zig 0.16+, but ghostel's native-module compile fallback needs exactly 0.15.2: ghostel pins ghostty 1.3.2-dev, whose build does requireZig(0.15.2), and 0.16's build-API changes break the dependency build scripts. So a plain pacman -S zig produces a zig that can't build ghostel.

install_zig_pin downloads zig-x86_64-linux-0.15.2.tar.xz from ziglang.org, verifies the sha256, extracts to /opt/zig-0.15.2, and symlinks /usr/local/bin/zig ahead of /usr/bin on PATH, where pacman -Syu can't bump it. I split the verify-and-install core (zig_install_from_tarball) out so it stays network-free and unit-testable: it refuses on a sha256 mismatch, a missing tarball, or a tree with no zig binary, and short-circuits when a correct install already exists.

ghostel's default path downloads a prebuilt module and needs no zig, so this only matters for the offline compile fallback. The pin needs a one-line bump (ZIG_VERSION + ZIG_SHA256) whenever ghostel moves to a newer ghostty.

Tests live in tests/zig-pin/: 7 cases covering extract+symlink, idempotency, sha256-mismatch refusal, missing tarball, and no-binary cleanup, run against the real function extracted from the script.

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