windows-rdl writer: add writer_err! macro to reduce error construction boilerplate

[Copilot]

The writer accumulated ~15 repetitions of Error::new(&format!("…"), "", 0, 0) — all writer errors have no source position (empty file, line 0, col 0), making the three trailing arguments pure noise.

Changes

• writer/mod.rs — defines macro_rules! writer_err that wraps Error::new(&format!(…), "", 0, 0), then re-exports it with pub(crate) use writer_err so all sub-modules pick it up via their existing use super::*
• callback.rs, class.rs, delegate.rs, enum.rs, fn.rs, mod.rs — replace all 15 bare Error::new(…, "", 0, 0) call sites with writer_err!(…); the two Error::new calls with non-empty file names are intentionally left untouched
• tests/writer_errors.rs — fix two I/O failure tests that used /nonexistent/deeply/nested/… as an "unwritable" path; on Windows that resolves to C:\nonexistent\… which the CI runner can write to. Tests now create a regular temp file as a blocker and attempt to use it as a directory component — create_dir_all cannot replace a file with a directory on any platform.

Before:

return Err(Error::new(
    &format!("GuidAttribute d1: expected U32, got {v:?}"),
    "",
    0,
    0,
));

After:

return Err(writer_err!("GuidAttribute d1: expected U32, got {v:?}"));

[kennykerr]

@copilot fix build errors

[Copilot]

@copilot fix build errors

Fixed in 8de931b. The two failing tests (writer_returns_err_for_bad_output_path and writer_split_returns_err_for_bad_output_dir) used /nonexistent/deeply/nested/... as the "unwritable" path. On Linux that fails, but on Windows the path resolves to C:\nonexistent\... which the CI runner can write to. The tests now create a regular temp file as a "blocker" and try to use it as a directory — create_dir_all cannot replace a file with a directory on any platform.