fix: remove tracked Zig build artifacts, update gitignore

Remove 19 tracked .zig-cache/ and zig-out/ files from runtime/shim/.
Add global (unanchored) .zig-cache/ and zig-out/ patterns to gitignore
so artifacts at any depth are ignored.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

.gitignore

@@ -96,3 +96,7 @@ htmlcov/
 # ReScript/OCaml compiler artifacts
 *.cmti
 *.cmi
+
+# Zig build artifacts (global)
+.zig-cache/
+zig-out/