feat(ergonomics): PathLike + PERCENT_40 typo + Slide.background.element fix (modernization Phase 1)

[MHoroszowski]

Modernization Phase 1 — PathLike + PERCENT_40 typo + Slide.background.element fix

Refs #29. Phase 1 of the Modernization & Ergonomics epic. Bundles three small, mostly-orthogonal wins the issue called out as "trivial cherry-picks" plus a long-standing correctness bug that pollutes the power-user surface.

What this PR adds / fixes

1. pathlib.Path / os.PathLike support across the API

Closes upstream PR scanny#1123. The four entry points users most commonly hit with a Path now accept it without a TypeError:

from pathlib import Path
from pptx import Presentation
from pptx.util import Inches

prs = Presentation(Path("deck.pptx"))             # ← used to TypeError
prs.save(Path("out.pptx"))                         # ← used to TypeError
slide.shapes.add_picture(Path("logo.png"), ...)    # ← used to AttributeError on .seek
Image.from_file(Path("logo.png"))                  # ← used to AttributeError on .seek

Each accepts any os.PathLike[str] (pathlib.Path, custom subclasses with __fspath__, etc.) by coercing via os.fspath(...) at the boundary. Existing str and file-like-object callers are unaffected.

2. MSO_PATTERN_TYPE.PERCENT_40 typo fixed

Closes upstream scanny#1131. The enum member was misspelled ERCENT_40 (missing leading P). Renamed to the correct PERCENT_40 with no value change (xml_value remains pct40, integer value remains 6). Code that referenced the broken name needs to update — the broken name is gone deliberately, callers will get a clear AttributeError.

3. slide.background.element returns the <p:bg> element

Closes upstream issue scanny#1126. Previously slide.background.element (and the private ._element) returned the parent <p:cSld> element instead of the actual <p:bg> background. Power users introspecting the XML now get the right node. The <p:bg> is materialized on construction (matching the legacy destructive behavior of accessing .fill); since slide.background is a lazyproperty, this only fires on first slide-background access, not on slide load.

Out of scope (Phase 2 follow-up)

• Font.color mutate-on-read (closes upstream Font.color getter mutates XML by inserting <a:solidFill/>, breaking theme color inheritance scanny/python-pptx#1111, Accessing @LazyProperty .color on Text.Font causes unintentional side effects scanny/python-pptx#1074) — the getter currently materializes <a:solidFill> on access, which means READING a font's color modifies the file. The fix requires a lazy ColorFormat wrapper that delays solidFill creation until the setter actually fires; that's a larger refactor that needs its own PR with careful review against the existing color/fill test suite.
• collections.abc import sweep (closes upstream Compaitibility script no longer breaks to collections changes in 3.10 scanny/python-pptx#771) — no remaining offenders in our fork's source tree (the upstream PR was authored against an older codebase). If any creep in via vendored fixes, Phase 2 will sweep.
• Dev-tooling modernization (uv, pyright strict, pytest-syrupy) — covered in the issue body but deserves its own PR per the issue's own "patch 1 / 2 / 3" framing.

Test coverage

• 15 new unit tests in tests/test_modernization_phase1.py:
  • 4 tests on Presentation() accepting Path/str/BytesIO/PathLike-subclass
  • 2 tests on Presentation.save(Path) round-trip
  • 1 test on add_picture(Path)
  • 3 tests on Image.from_file(Path|str|stream)
  • 2 tests on the PERCENT_40 typo fix
  • 3 tests on slide.background.element (returns <p:bg>, fill still works, RGB round-trip survives)
• 4 new behave scenarios in features/modernization-phase1.feature
• UAT uat_modernization_phase1.py (untracked per repo §6) — programmer ergonomics, no visual change to inspect, so the script prints per-fix verification. UAT signoff: ✓ — also confirmed in the wild with a real add_picture(Path) traceback that this PR resolves.

Verification

$ python3 -m pytest tests/ -q | tail -3
3237 passed in 4.54s

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

$ python3 -m behave features/ --no-color | tail -3
1003 scenarios passed, 0 failed, 0 skipped
3010 steps passed, 0 failed, 0 skipped