fix: add notesMasterIdLst element when creating notes master

[robbybrodie]

Summary

When python-pptx creates a notes master on first access to slide.notes_slide, it correctly creates the NotesMasterPart and adds the relationship in the .rels file, but omits the corresponding <p:notesMasterIdLst> reference from presentation.xml. Without this element, OOXML consumers cannot discover the notes master from the presentation element, even though the relationship and part exist in the package.

This causes Apple Keynote (and potentially other OOXML consumers) to fail to recognize the notes master, breaking speaker notes when opening python-pptx-generated files.

What this PR does

• Adds CT_NotesMasterIdList and CT_NotesMasterIdListEntry element classes with proper ZeroOrOne/RequiredAttribute declarations
• Registers both element classes in the oxml element class lookup
• Adds a ZeroOrOne declaration for notesMasterIdLst on CT_Presentation with the correct successor sequence per the OOXML spec
• Updates PresentationPart.notes_master_part to populate the notesMasterIdLst element with the relationship ID after creating the notes master relationship
• Updates the existing unit test to verify the new element is created correctly

Before (missing element)

<p:presentation>
  <p:sldMasterIdLst>...</p:sldMasterIdLst>
  <!-- notesMasterIdLst is absent even though notesMaster.xml exists in the package -->
  <p:sldIdLst>...</p:sldIdLst>
</p:presentation>

After (element present)

<p:presentation>
  <p:sldMasterIdLst>...</p:sldMasterIdLst>
  <p:notesMasterIdLst>
    <p:notesMasterId r:id="rId8"/>
  </p:notesMasterIdLst>
  <p:sldIdLst>...</p:sldIdLst>
</p:presentation>

Test results

Unit tests (pytest): 2700 passed, 0 failed

Acceptance tests (behave): 54 features, 973 scenarios, 2914 steps — all passed

The existing acceptance test in prs-presentation-props.feature (Presentation.notes_master with no notes master) already exercises the code path this fix modifies — creating a notes master when none exists. No new acceptance test was needed.

Manual verification:

• Generated PPTX contains the notesMasterIdLst element in presentation.xml
• Generated PPTX with speaker notes opens correctly in Apple Keynote with notes intact

Closes #1051