Move private Sendspin player methods to the bottom of the class (#5657)

# What does this implement/fix?

The `SendspinPlayer` class had public and private methods interleaved,
which the repo rule in AGENTS.md ("private methods at the bottom, public
at the top") disallows. It was grandfathered in
`scripts/lint_baselines/private_methods_not_last.txt` at 12 violations.

This reorders the class so all public methods come first, and drops the
file from the lint baseline. No behaviour change — every method body,
signature, decorator and docstring is untouched, and nothing was added
or removed.

The diff is large but is a pure move; `git diff --color-moved` makes it
readable.

- Moved all private methods of `SendspinPlayer` below the public ones
- Kept related helpers grouped: the artwork helpers together, and the
beat-schedule helpers (including the `_BEAT_RETRY_*` constants) together
- Removed `music_assistant/providers/sendspin/player.py` from the
method-order lint baseline (12 → 0)

`SendspinBasePlayer` and `SendspinVisualizerPlayer` in the same file
already complied and were left alone.

**Related issue (if applicable):**

- related issue <link to issue>

## Types of changes

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- [ ] Bugfix (non-breaking change which fixes an issue) — `bugfix`
- [ ] New feature (non-breaking change which adds functionality) —
`new-feature`
- [ ] Enhancement to an existing feature — `enhancement`
- [ ] New music/player/metadata/plugin provider — `new-provider`
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected) — `breaking-change`
- [ ] Refactor (no behaviour change) — `refactor`
- [ ] Documentation only — `documentation`
- [x] Maintenance / chore — `maintenance`
- [ ] CI / workflow change — `ci`
- [ ] Dependencies bump — `dependencies`

## Checklist

- [x] The code change is tested and works locally.
- [x] `pre-commit run --all-files` passes.
- [x] `pytest` passes, and tests have been added/updated under `tests/`
where applicable.
- [ ] For changes to shared models, the companion PR in
`music-assistant/models` is linked.
- [ ] For changes affecting the UI, the companion PR in
`music-assistant/frontend` is linked.
- [x] I have read and complied with the project's [AI
Policy](https://github.com/music-assistant/.github/blob/main/AI_POLICY.md)
for any AI-assisted contributions.
- [ ] I have [raised a PR against the documentation
repository](https://github.com/music-assistant/music-assistant.io/blob/main/CONTRIBUTING.md)
targeting the main or beta branch as appropriate.