fix: remove tracked Zig build artifacts, update gitignore

Remove 5 tracked .zig-cache/ files from src/interface/ffi/.
Replace path-specific zig patterns with global (unanchored) .zig-cache/
and zig-out/ patterns to match at any depth.

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

Author: hyperpolymath

.gitignore

@@ -53,11 +53,9 @@ __pycache__/
 /lib/
 /proof/
 
-# Zig (new layout under src/interface/ffi/)
-/zig-cache/
-/zig-out/
-/src/interface/ffi/zig-cache/
-/src/interface/ffi/zig-out/
+# Zig build artifacts (global — matches any depth)
+.zig-cache/
+zig-out/
 
 # Rust (new layout under src/rust/)
 /src/rust/target/