First slice of issue #11 (slide CRUD epic). Lands the in-deck CRUD
operations users have been asking for since 2014; full duplicate(),
cross-deck append_from(), and Sections support stay deferred to
Phases 2–4 in the epic.

New public API
--------------
- `Slides.add_slide(slide_layout, index=None)` — `index=None`
  preserves existing append behavior (backwards compatible). Integer
  `index` inserts at that zero-based position; `index=len(self)`
  is a valid append, negatives raise `IndexError`.
- `Slides.move(slide, new_index)` — reorder a slide in the
  presentation's slide sequence. Raises `ValueError` for foreign
  slides, `IndexError` for out-of-range `new_index`.
- `Slides.remove(slide)` — drop a slide from the collection. The
  presentation→slide relationship is removed; the underlying part
  falls out on save. Image and other media parts shared with
  surviving slides are preserved (covered by integration test).
- `Slide.delete()` — convenience alias for
  `prs.slides.remove(self)`.

API shape mirrors the existing `SlideLayouts.remove(slide_layout)`
shape rather than upstream PR scanny#1029's
`remove_slide(slide_id)` — instance-keyed is more Pythonic and
matches the rest of the library.

Internal additions
------------------
- `CT_SlideIdList.insert_sldId_at(rId, idx)` — index-aware sldId
  insertion, with `idx == len` allowed.
- `CT_SlideIdList.move_sldId_to(sldId, new_idx)` — reposition an
  existing sldId; no-op when the index already matches.
- `CT_SlideIdList.remove_sldId(sldId)` — bounded remove with a
  parent-of check.

Test coverage
-------------
- 32 new unit tests in `tests/test_slide_crud.py` covering oxml
  helpers, the Slides CRUD methods, the Slide.delete alias, and four
  full-pipeline round-trip integration tests (add-at-index, remove,
  delete, move). Plus a shared-image-preservation round-trip and a
  `index=len(slides)` boundary test added per advisor review.
- 5 new behave scenarios in `features/sld-crud.feature` exercising
  add-at-head, add-in-middle, move, remove, and Slide.delete.
- New UAT script (untracked) at `uat_slide_crud.py` builds a deck
  exercising every new entry-point, saves, reopens, and prints a
  per-slide read-back so the maintainer can compare in PowerPoint /
  Keynote per CLAUDE.md §6.

Tangential test-isolation fix
-----------------------------
`tests/opc/test_package.py:DescribePartFactory` mutates
`PartFactory.part_type_for[CT.PML_SLIDE]` to a class_mock and
never reverts it. The mock leaked into any later test that loaded
slides through `Presentation()` — caught here by the new
round-trip integration tests. Restored via a request finalizer;
isolated to that one test, no behavior change for the production
code path.

Verification
------------
```
$ python3 -m pytest tests/ -q | tail -3
3086 passed in 3.76s

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

$ python3 -m behave features/ --no-color | tail -3
986 scenarios passed, 0 failed, 0 skipped
2952 steps passed, 0 failed, 0 skipped
```

Refs #11

…e 2) (#34)

Closes the Phase 2 sub-task of the slide-CRUD epic (#11) and the
12-year-old most-commented upstream feature request scanny#132
in this fork.

API
---

- `Slides.duplicate(slide, index=None)` — collection-level operation,
  returns a `Slide`. `index=None` defaults to `source_index + 1`
  (immediately after the source). Raises `ValueError` if `slide` is
  not a member, `IndexError` if `index` is out of range.
- `Slide.duplicate(index=None)` — convenience alias delegating to
  `Slides.duplicate(self, index)`. Mirrors `Slide.delete()` →
  `Slides.remove(self)` from Phase 1.

Design
------

The implementation walks the source slide's rels and decides per
reltype whether to share the target part (image, media, slide-layout,
slide-master, theme — package-level dedup) or deep-copy it (chart,
OLE-object, embedded-package, notes-slide). For chart parts the chart
XML is `copy.deepcopy`'d but the embedded xlsx workbook is *blob-
copied* — the workbook is the chart's data and `<c:numCache>` values
in the chart XML mirror it; deepcopy of XML would not capture the
binary payload.

After deepcopy, the duplicated `<p:sld>` still references the source's
`rId`s. We walk every descendant element via `lxml.iter()` and
substitute any attribute whose name is in the OOXML relationships
namespace — catches `r:id`, `r:embed`, `r:link`, `r:pict`, `r:href`
in one pass without enumerating local-names (the local-name approach
silently misses `r:link` on linked images, which is the most common
real-world failure mode of community recipes).

Notes-slide handling addresses upstream gotcha scanny#961: a NotesSlidePart
back-references its parent slide via `RT.SLIDE`. The duplicate's
notes-slide MUST point at the new slide, not the source. The new
helper `duplicate_notes_slide_for(src, new)` rewires the back-rel
and is invoked AFTER the new slide part is registered in presentation
rels.

Comments parts (`RT.COMMENTS` / `RT.COMMENT_AUTHORS`) are dropped on
duplicate by an explicit reltype filter — comments support is out of
scope for Phase 2 of #11. A test documents the drop so the policy
doesn't silently change.

Test coverage
-------------

- `tests/test_slide_duplicate.py` — 39 new tests across 7 describe
  classes covering the API surface (existence, signature, raises,
  index variants, alias delegation), the part-graph invariants (new
  unique partname / rId / sldId, layout-part shared with source,
  modification isolation, image-dedup invariant unchanged), the
  notes-slide path (own copy, back-ref rewired, isolation), the
  defensive XPath `r:*` check (no unmapped rIds), and round-trip
  through save/reopen.
- `features/sld-duplicate.feature` + 7 new step impls — 4 acceptance
  scenarios for default-index, explicit-index, alias, and IndexError
  paths.

Verification
------------

```
$ python3 -m pytest tests/ -q
3125 passed in 4.02s
$ python3 -m ruff format src tests
203 files left unchanged
$ python3 -m ruff check src tests
All checks passed!
$ python3 -m behave features/ --no-color
990 scenarios passed, 0 failed, 0 skipped
```

UAT script `uat_slide_duplicate.py` (untracked at repo root per
CLAUDE.md §6): builds a 3-slide deck with a textbox, a picture, and
speaker notes; exercises both `Slides.duplicate` and
`Slide.duplicate`; round-trips; prints structural summary
(image_part_count = 2 before AND after — dedup invariant holds).

Out of scope (deferred per epic split):
- `Presentation.append_from` — Phase 3 (cross-deck rels).
- `Presentation.sections` — Phase 4 (`<p14:sectionLst>`).

Refs #11.

Co-authored-by: Matthew Horoszowski <mhoroszowski@Winton.lan>