chore: strengthen .gitignore for build artifacts

Add patterns to prevent accidental commit of:
- build_er*.txt (error logs)
- root.zig (generated root file)
- l0_*.zig through l4_*.zig (generated layer files)

.gitignore

@@ -21,9 +21,16 @@ libqvl_ffi.a
 # Build Error Logs
 build_err*
 build_errors.txt
+build_er*.txt
 
 # Random build artifacts
 root
+root.zig
+l0_*.zig
+l1_*.zig
+l2_*.zig
+l3_*.zig
+l4_*.zig
 
 # Operational Reports & Stories
 REPORTS/