chore(cli): gitignore Zig build artifacts, rm cached cache+out

Remove .zig-cache/ and zig-out/ from tracking; add ffi/zig/.gitignore.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: hyperpolymath

verisim-modular-experiment/ffi/zig/.gitignore

@@ -0,0 +1,3 @@
+# SPDX-License-Identifier: PMPL-1.0-or-later
+.zig-cache/
+zig-out/