feat: HandoutMaster + add_text_watermark helper — Phase 5, closing #20

[MHoroszowski]

Phase 5 of #20 — HandoutMaster + watermark helper (closes the epic)

Ships the final two pieces of issue #20: a public HandoutMaster Python class so users can read and toggle handout-master visibility settings without dropping into XML, and a SlideMaster.add_text_watermark() helper that wraps the common "DRAFT every slide" pattern into one call.

What this PR completes

# read existing handout master
prs = Presentation("with_handout.pptx")
hm = prs.handout_master                 # raises ValueError when absent
hm.show_footer = False                  # _HeaderFooterVisibility from Phase 2

# add a watermark to every slide using a master
sm = prs.slide_masters[0]
wm = sm.add_text_watermark("DRAFT", transparency=0.7)
prs.save("out.pptx")

Open out.pptx in PowerPoint or Keynote: every slide using the modified master shows a centered, 72pt, ~70%-transparent "DRAFT" — and the handout-master footer toggle round-trips.

Changes

• pptx.slide.HandoutMaster (NEW) — _HeaderFooterVisibility + _BaseMaster, mirrors NotesMaster structure. Inherits placeholders, shapes, and the four show_* properties.
• pptx.parts.slide.HandoutMasterPart (NEW) — BaseSlidePart mirror of NotesMasterPart, without create_default() / _new() / _new_theme_part(). Read-only.
• pptx.parts.presentation.PresentationPart — new handout_master_part and handout_master lazyproperties. No auto-create — raises ValueError("presentation has no handout master; auto-create is deferred …") when the relationship is absent. Chained from KeyError so the original lookup failure is preserved.
• pptx.presentation.Presentation.handout_master (NEW property) — pass-through, matches notes_master shape.
• pptx.shapes.shapetree.MasterShapes.add_textbox (NEW, three lines) — _BaseShapes only exposed add_textbox on SlideShapes/LayoutShapes. The watermark helper needs it on the master.
• pptx.slide.SlideMaster.add_text_watermark(text, *, font_size=Pt(72), transparency=0.7, font_name="Calibri") (NEW) — centered textbox at 6in × 1.5in on the standard 10in × 7.5in slide; mid-gray (#808080) text; routes transparency through the existing FillFormat.transparency from PR feat(dml): add transparency support for solid fills and colors (closes #17) #30. Returns the Shape for fine-tuning.
• pptx.__init__ — registered HandoutMasterPart against CT.PML_HANDOUT_MASTER in content_type_to_part_class_map.

Out of scope (deliberate)

• Auto-create of handout master when absent — needs a handoutMaster.xml template file plus theme wiring. Raising ValueError with a clear message is the explicit Phase 5 contract.
• add_image_watermark — text only; users who need a picture watermark call master.shapes.add_picture(...) directly today.
• MSO_WATERMARK_PRESET enum / canned styles — string text + numeric transparency, that's the whole API surface.

Verification (local, CPython 3.14.4)

python3 -m pytest tests/ -q                       → 3651 passed in 5.25s (+19 vs Phase 4)
python3 -m ruff check src tests                   → All checks passed!
python3 -m ruff format --check src tests          → 216 files already formatted
python3 -m behave features/ --no-color            → 1048 scenarios, 0 failed
python3 uat/uat_headers_footers_phase5.py         → PASS (absent-path ValueError + watermark text + transparency round-trip)

19 new test cases:

• DescribeHandoutMaster (6 cases): class existence, mixin chain, show_* round-trips.
• DescribeHandoutMasterPart (4 cases): instantiation, handout_master lazyproperty, content-type registration, no create_default.
• DescribePresentationPart extension (3 cases): handout_master_part returns related part, raises ValueError when absent, error message contains "no handout master".
• DescribePresentation extension (1 case): Presentation.handout_master pass-through.
• DescribeSlideMasterAddTextWatermark (4 cases): returns Shape, text matches argument, default transparency, centered position.

Epic close summary

Five phases shipped:

Phase	PR	Surface
1	#48	CT_HeaderFooter + CT_HandoutMaster + CT_TextField attrs (OOXML wrappers)
2	#49	Slide.footer/has_footer/has_slide_number/has_date/date_text + show_* on layout/master/notes-master
3	#50	_Paragraph.add_field() + _Field class + CT_TextField.text setter + control-char escape
4	#51	_Paragraph.fields discovery accessor
5	this	HandoutMaster + HandoutMasterPart + Presentation.handout_master + SlideMaster.add_text_watermark

Test count: 3514 (issue-#20 epic start) → 3651 (epic close). Behave: 1048 / 0 throughout. Zero regressions across all five phases.

Closes #20.