chore: extract SMIL writer to standalone PR #74

Remove anim-smil-writer.ts and its tests from this branch. The SMIL
PPTX animation export module is zero-dependency and has been moved to
a separate PR (feat/smil-pptx-export → #74) for independent review.

Vitetest config updated to remove the SMIL environment entry.

Signed-off-by: Jacob <jacob.hxx.cn@outlook.com>
Signed-off-by: Jacobinwwey <jacob.hxx.cn@outlook.com>

Author: Jacobinwwey