Zig ergonomics

[treeform]

• Emit Zig Error!T wrappers for Nim APIs that can raise, so Zig callers use normal try / catch flow.
• Check the generated checkError() flag before converting raw ABI return values back into Zig values.
• Omit explicit callconv(.C) on extern fn declarations for Zig version compatibility, and update Zig tests to use std.debug.print.
• Update the Pixie Zig test to exercise the new error-union API and assert the bad-color error path.