[Epic] Slide Masters, Layouts & .potx Templates

[MHoroszowski]

Problem

Slide masters and layouts are visible-but-frozen. Users can read the master/layout shape lists but cannot programmatically add new layouts (scanny/python-pptx#1091, #413), apply layouts from one master to another (#1028), or read .potx template files (#1070, #1095) — Presentation() raises on .potx because the content-type allowlist doesn't include .template. Adding shapes/pictures to a Slide Master is impossible (#575, 11c). Content placeholders are partial (#333, 12c).

Sub-features

• Presentation() accepts .potx files (one-line content-type allowlist fix)
• Presentation.save_as_potx(path) — write a template by changing the content-type
• SlideMasters.add_layout(name, copy_from=None) — add a new <p:sldLayout> part with rels
• SlideLayouts.copy_from(other_layout) — duplicate a layout including placeholders and shapes
• SlideMaster.shapes.add_* — author shapes/pictures directly on a master (not inherited via layout)
• SlideLayout.placeholders.add(idx, type, ...) — programmatically add placeholders
• Apply a layout from master_a to a slide that's inheriting from master_b (cross-master rel resolution)
• Improved content-placeholder semantics: chart-into-placeholder (#199, 11c)
• SlideMaster.get_layout(slide_layout_id, default=None) (scanny/python-pptx#269)

Prior art

• Open PRs: scanny/python-pptx#1091 add_slide_layout (wkpark — clean, ~111 LOC). scanny/python-pptx#1071 POTX read (9021007 — one-line allowlist fix). scanny/python-pptx#1044 textbox-to-layout. scanny/python-pptx#604 object placeholder (WIP). scanny/python-pptx#292 layout-names refactor (21k LOC — review carefully, may be too sweeping).
• Forks:
  • wkpark/python-pptx slidelayout branch.
• User issues this would close: #199, #269, #333, #413, #575, #603, #608, #804, #824, #864, #1028, #1044, #1070, #1095, #1109.
• OOXML: PresentationML §19.3.1.42 <p:sldLayout>, §19.3.1.44 <p:sldMaster>, content-type application/vnd.openxmlformats-officedocument.presentationml.template.main+xml (POTX).
• Code paths: src/pptx/parts/slidemaster.py, src/pptx/parts/slidelayout.py, src/pptx/slides.py (SlideMasters/SlideLayouts), src/pptx/__init__.py (content-type registration), src/pptx/oxml/presentation.py.

Acceptance criteria

• Presentation("template.potx") opens without error.
• pres.save_as_potx("out.potx") produces a file PowerPoint recognizes as a template.
• master.add_layout("Three Columns") creates a new layout that's selectable in PowerPoint's layout pane.
• 20+ unit tests + 6 behave scenarios.

Effort: M