chore(gitignore): add **/.zig-cache-global/ pattern (#136)

## Summary

- Extends `.gitignore` to cover `.zig-cache-global/` (only
`**/.zig-cache/` was present)
- Prevents recurrence of the cache-pollution that necessitated closing
#135

## Why

PR #135 inadvertently staged ~98 files under `.zig-cache-global/`, the
cache directory Zig uses when `ZIG_GLOBAL_CACHE_DIR` is set. The
existing ignore pattern was for the default `.zig-cache/` only. This
one-line addition closes the gap.

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

Author: hyperpolymath

.gitignore

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