refactor: use pathlib for file paths and enable ruff PTH rules

[bluetoothbot]

What

Convert os.path / bare open() usage to pathlib.Path across the repo, then add the PTH (flake8-use-pathlib) rule family to ruff's select list.

Why

Continues the incremental ruff-rule-family expansion from #721. PTH was the cheapest remaining clean candidate — 24 violations tree-wide, all mechanical, zero pre-existing exceptions needed.

How

• src/dbus_fast/_private/address.py: two open(...) calls (machine-id + dbus session info file) → Path(...).open(). Behaviour unchanged; .pxd untouched (no signature change). Sync I/O semantics preserved — the existing XXX: this will block comment still applies.
• docs/source/conf.py, examples/*.py, bench/unmarshall_getmanagedobjects.py: sphinx/example sys.path bootstrap and fixture loaders.
• tests/test_address_parser.py, tests/test_introspection.py, tests/test_marshaller.py: fixture file reads.
• pyproject.toml: add PTH to [tool.ruff.lint] select.

Testing

• dbus-run-session -- poetry run pytest --timeout=5 -q → 363 passed, 20 skipped, coverage 85.35%.
• poetry run ruff check . → clean.
• poetry run ruff format --check . → 88 files already formatted.

---

Quality Report

Changes: 13 files changed, 31 insertions(+), 24 deletions(-)

Code scan: clean

Tests: failed (FAILED)

Branch hygiene: clean

Generated by Kōan post-mission quality pipeline

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.38%. Comparing base (f5398eb) to head (d1a98d8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #738   +/-   ##
=======================================
  Coverage   91.38%   91.38%           
=======================================
  Files          29       29           
  Lines        3621     3622    +1     
  Branches      623      623           
=======================================
+ Hits         3309     3310    +1     
  Misses        186      186           
  Partials      126      126           

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