ci: test Python 3.15 and document compatibility blockers

[tobiasraabe]

Add experimental Python 3.15 test jobs on Linux, macOS, and Windows, with only those jobs allowed to fail while compatibility is investigated. Windows 3.15 follows the existing notebook exclusion, matching the nbmake dependency marker. Include the linked changelog entry.

Confirmed CI blockers, documented with job links in .github/PYTHON315.md:

• Linux: locked rpds-py 0.25.1 uses PyO3 0.25.0, which rejects Python 3.15. This comes through the notebook dependency chain.
• Windows: locked cachebox 5.2.3 uses PyO3 0.27.1, which rejects Python 3.15. Deepdiff 9.1.0 requires cachebox<6, preventing a simple upgrade to the release with 3.15 wheels.
• Locked aiohttp and greenlet lack 3.15 wheels but successfully build from source on Windows CI; local compiler failures are recorded separately.
• Locked pywin32 310 prevents Windows notebook coverage; newer wheels exist and need follow-up validation.

Keep the lockfile and supported-version classifiers unchanged. The highest-resolution job remains on 3.14 until 3.15 can run the applicable suites successfully.

Validation: changed-file hooks and zizmor passed. Python 3.15.0 installed successfully. Linux and Windows CI failed during dependency installation before test collection; macOS was still running when recorded.