chore(python): bump docx-scalpel to 0.1.0a3

[JSv4]

Bump python/pyproject.toml version 0.1.0a2 → 0.1.0a3 so the next docx-scalpel PyPI release can ship with all the Python work that landed since the original a2 publish on 2026-05-26.

Why a3 instead of re-tagging a2

PyPI already has docx-scalpel==0.1.0a2 from commit 109d98b (just the version bump). Since then we shipped:

• Annotation write surface in feat: annotation write surface (DocxSession + WASM + docx-scalpel) #204 (Python wrapper)
• Block-metadata reads in feat(session): block metadata read surface (GetBlockMetadata/GetListMembership/GetSectionInfo) #205 (Python wrapper)
• re-export block-metadata types from package root fix

PyPI is immutable, so the new code must ship under a new version. a3 keeps the alpha cadence.

After merge, I'll tag docx-scalpel-v0.1.0a3 on main → the python-publish.yml workflow auto-publishes to PyPI.

Test plan

• One-line version bump; no code changes.