chore: remove tracked Zig cache and fix .gitignore patterns

Unanchor zig-out/, zig-cache/, target/, _build/ patterns to match
nested monorepo paths. Add .zig-cache/ pattern.
Removes 18 tracked Zig build cache files from overlay-protocol/.

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

Author: hyperpolymath

.gitignore

@@ -10,14 +10,15 @@ Thumbs.db
 .idea/
 .vscode/
 
-# Build
-/target/
-/_build/
+# Build (unanchored to match nested monorepo paths)
+target/
+_build/
 /build/
 /dist/
 /out/
-/zig-out/
-/zig-cache/
+zig-out/
+zig-cache/
+.zig-cache/
 
 # Dependencies
 /node_modules/