chore(gitignore): add **/.zig-cache-global/ to ignore Zig global cache

The existing pattern `**/.zig-cache/` doesn't catch `.zig-cache-global/`,
which is what Zig uses when ZIG_GLOBAL_CACHE_DIR is set. PR #135 was
closed because a session committed ~98 cache files under this path; this
prevents recurrence.

Author: hyperpolymath

.gitignore

@@ -22,6 +22,7 @@ build/
 # Zig
 **/zig-out/
 **/.zig-cache/
+**/.zig-cache-global/
 
 # Coq / Rocq
 *.vo