feat(a11y): reading_order + accessibility_issues + is_hidden_from_accessibility (Phase B, closes #22)

[MHoroszowski]

Accessibility Phase B — reading_order + accessibility_issues + is_hidden_from_accessibility

Closes #22. Phase B of the Accessibility epic. Phase A (alt_text, alt_title, is_decorative) shipped in #31. With this PR the items the issue called out are all covered, modulo Microsoft Accessibility Checker's deeper diagnostics (color contrast, font size, etc.) which are outside python-pptx's scope.

Phase A regression fix bundled in

PR #31's <adec:decorative> extension shipped with the wrong GUID for the <a:ext uri="..."> wrapper. PowerPoint's accessibility-rendering pipeline only recognizes the extension under one specific URI, and ours didn't match — so the XML round-tripped within python-pptx (unit tests passed) but PowerPoint silently ignored it and the "Mark as Decorative" checkbox in the Alt Text pane stayed unchecked on reopen.

	URI
Before (PR #31, wrong)	{FF2B5EF4-FFF2-40B4-BE49-F238E27FC236}
After (matches PowerPoint exactly)	{C183D7F6-B498-43B3-948B-1728B52AA6E4}

Confirmed empirically by diffing a maintainer-authored deck (where the checkbox was set in the UI, then saved) against a python-pptx-emitted deck — the only material difference was the GUID. Both is_decorative getter and setter, plus their unit tests, are updated to the correct GUID.

What this PR adds

# Convenience alias for is_decorative
shape.is_hidden_from_accessibility = True
shape.is_hidden_from_accessibility  # → True

# Reading order — z-order of shapes on the slide
slide.shapes.reading_order               # → tuple of shapes
slide.shapes.reading_order = (c, a, b)   # → reorders the spTree

# Lint helper — shapes lacking alt text and not marked decorative
slide.shapes.accessibility_issues()      # → list of BaseShape

Public API

• Shape.is_hidden_from_accessibility (boolean, read/write). Convenience alias for is_decorative. Some accessibility documentation and third-party tools use the wording "hidden from accessibility" for the same flag; this property exists so the API reads naturally for either audience. Backed by the same <adec:decorative val="1"/> extension.

• Slide.shapes.reading_order (read/write).

  • Getter returns a tuple of shapes in z-order — the order screen readers will narrate them on a slide that does not declare an explicit <p:tabLst>.
  • Setter accepts a permutation of the slide's existing shapes and reorders the underlying <p:spTree> to match. Raises ValueError on wrong length or unknown shapes.

• Slide.shapes.accessibility_issues() → list[BaseShape]. Lint helper. Returns shapes that lack alt text (neither alt_text nor alt_title is set) AND are not marked decorative. Returned in reading order. A fully-tagged slide reports []. Treat it as a fast first-pass — color-contrast / font-size / explicit-tab-order checks are not covered.

Out of scope (Phase C if it ever surfaces)

• <p:tabLst> (the OOXML element for explicit reading order on a per-slide basis) is rare in practice — PowerPoint's UI doesn't expose it, and the OOXML spec calls out that the fallback for reading order is shape z-order. We deliberately model reading_order as z-order rather than emitting <p:tabLst>. This keeps the API simple and works correctly for the overwhelming majority of decks. If a real-world need for explicit <p:tabLst> emerges, that's a Phase C addition that would preserve the existing API as the no-tabLst fallback.

Test coverage

• 20 new unit tests in tests/test_a11y_phase_b.py:
  • 5 tests on the is_hidden_from_accessibility alias including save/reopen round-trip
  • 9 tests on reading_order (getter, setter, round-trip, no-op, raise on wrong length, raise on unknown shape)
  • 6 tests on accessibility_issues() (flag-untagged, pass alt-text, pass alt-title, pass decorative, fully-tagged-slide empty, return-in-reading-order)
• 4 new behave scenarios in features/a11y-phase-b.feature (alias, reading_order getter, reading_order setter, lint helper)
• Existing tests/shapes/test_base.py is_decorative cases updated to the corrected GUID — semantic test surface unchanged.
• UAT uat_a11y_phase_b.py (untracked per repo §6) builds a 2-slide deck. UAT signoff: ✓ — verified end-to-end that PowerPoint's "Mark as Decorative" checkbox now reads as checked on reopen, comparing against a maintainer-authored reference deck.

Verification

$ python3 -m pytest tests/ -q | tail -3
3242 passed in 4.42s

$ ruff check src tests | tail -3
All checks passed!

$ python3 -m behave features/ --no-color | tail -3
1003 scenarios passed, 0 failed, 0 skipped
3011 steps passed, 0 failed, 0 skipped