Enforce `from __future__ import annotations` everywhere (bring coverage 84% → 100%)

[loadfix]

Problem

python-docx has from __future__ import annotations in 145 of 172 source files (~84%). The sibling python-pptx has it in 119 of 128 (~93%). The gap is unexplained and contributes to subtle type-annotation inconsistency — some files use PEP 604 str | None freely, others stick to Optional[str].

Expected fix

Enable ruff rule I002 (required-from-future-import) or the equivalent isort setting in [tool.ruff.lint.isort]:

required-imports = ["from __future__ import annotations"]

Run ruff check --fix to insert the directive into the 27 files currently lacking it.

Benefit

• Contributors stop wondering "should I use str | None here or Optional[str]?"
• Consistency with python-pptx (which should receive the same treatment as a follow-up)
• Tiny forward-compat benefit for Python 3.14+ stringified annotation changes

Surfaced by: 2026-05-05 consistency audit (item 8).