Add ChangePoints parity tests

[Gerrrr]

Add a hand-authored sparse reference case and Hypothesis-generated sparse histories for ChangePointsByTime and ChangePointsByMetric.

Read parity covers iteration and indexing, metric enumeration and selection, per-metric reads, timestamp and commit lookup, and pivot/by_time/by_metric conversions. Mutation coverage exercises copy isolation, shared conversion views, append/extend construction, and invalid or out-of-order boundaries.

The tests exposed and this commit fixes three parity bugs:

• ChangePoints.select_metrics() retained empty timestamp rows and disagreed with the metric view for missing or invalid metrics. It now validates inputs, raises consistent TypeError/KeyError exceptions, and omits empty groups.
• ChangePointsByMetric.pivot() structurally contaminated a source metric bucket when merging metrics at the same timestamp. It now builds shallow group views, so conversion itself is non-mutating while shared ChangePoint semantics remain intact.
• ChangePointsByMetric.extend() stored a complete multi-metric group in every metric bucket, causing duplicate metrics during later pivots. It now stores the matching per-metric projection, consistent with append().

In addition, this PR documents that conversions are shared views and that copy().pivot() provides isolation. This PR also adds Hypothesis to the development dependencies.

[Copilot]

Pull request overview

Adds parity coverage and fixes behavior inconsistencies between time- and metric-oriented change-point views.

Changes:

• Adds deterministic and Hypothesis-generated parity tests.
• Fixes metric selection, pivot mutation, and metric projection behavior.
• Documents shared conversion views and adds Hypothesis dependencies.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 1 comment.

File	Description
otava/change_point_divisive/base.py	Fixes view parity and documents copy semantics.
tests/change_point_classes_test.py	Adds parity and mutation tests.
pyproject.toml	Adds Hypothesis development dependency.
uv.lock	Locks Hypothesis and transitive dependencies.

Suppressed comments (1)

otava/change_point_divisive/base.py:708

• This validation only checks the outer container. A list containing a non-string metric bypasses the new TypeError path and later raises KeyError (or an incidental unhashable-type error), so the two views still do not expose a consistent invalid-input contract. Validate list elements here as well.

        if not isinstance(m, list):
            if not isinstance(m, str):
                raise TypeError(
                    "ChangePoints.select_metrics() takes as argument a str or a list of str."
                )

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.

[Gerrrr]

@henrikingo I am going to merge this and #171 to unblock #169 and further work. Feel free to take a look async / I will be happy to address any feedback in follow-up PRs.

[henrikingo]

Thanks, I'm catching up, happy to see so much PR activity. I'll give an update on the mailing list soon. But for now I try to catch up with reviewing and responding to PR and issues.