fix: correct Diagnostic Python 3.15 exception

[stefan-jansen]

Summary

• replace the obsolete Diagnostic SciPy exception with the current PyArrow blocker
• constrain the replacement to the published 0.1.2 release and retain the 2026-09-30 expiry
• add proposed Decision 0003 with upstream and library evidence

Verification

• uv run ruff check .
• uv run ruff format --check .
• uv run ty check
• uv run pytest -q (80 passed)
• uv run mkdocs build --strict
• pre-commit run --all-files

Review requirement

Decision 0003 and the executable exception record remain marked pending review. Before this PR can be made ready, the decision status and approver must be updated in a follow-up commit.

Tracks #2.

[stefan-jansen]

Pending maintainer decision. Local and hosted qualification pass at the current head. The proposed replacement records PyArrow 25.0.1 as the actual CPython 3.15 blocker for Diagnostic 0.1.2, limits the exception to >=0.1.2,<0.1.3, and retains the 2026-09-30 expiration. The decision and approver fields remain unaccepted until explicit review.