chore: prepare 0.1.9 maintenance candidate

[stefan-jansen]

Prepare the 0.1.9 maintenance candidate without publishing it.

Only the package's dynamic source version changes from 0.1.8 to 0.1.9. Dependency constraints, the lock, APIs and the historical migration comparisons remain unchanged. This administrative version change does not need a separate issue under the contribution policy.

The candidate includes the verified numerical, Polars and configuration repairs from #99–#103, the GitPython security fix from #93, documentation link-check reliability from #105, migration guides from #95, workflow pins from #96 and contribution routes from #98. The original @quantum-like contribution and its attribution remain in those owning PRs.

At reviewed head b1a5de1:

• uv lock --check, Ruff check/format, ty and all-file pre-commit hooks passed.
• uv run --locked --extra ml pytest tests/ -q -n auto --timeout 120 --disable-warnings: 5484 passed, 7 skipped.
• Strict MkDocs, exact diagnostic/0.1.9/commit identity and 9665 README/rendered links with 20 external destinations passed.
• Built 0.1.9 wheel/sdist metadata and contents passed. Isolated wheel imports, README quick start and all 11 task-documentation pages passed.
• An initial local test run correctly caught stale editable metadata after the version bump. Reinstalling the editable package restored source/installed version agreement; no test or production-code workaround was added.

Current-head hosted supported CI and qualification must also pass before merging. Python 3.15 remains explicitly deferred under #45; support remains >=3.12,<3.15.

No release workflow, tag, PyPI publication or canonical website deployment is requested or executed by this PR. Those require a separate user decision after the final main candidate is verified.

AI assistance: Codex prepared the version-only change and ran the checks. The maintainer remains accountable for the change.

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