chore: evaluate `xarray_plotly` dependency strategy for `fluxopt-plot`

[FBumann]

Summary

Decide whether fluxopt-plot should inline xarray_plotly or keep it as a declared ~= dependency. Both packages are maintained by the same author, so either approach is viable.

Options

Option A: Keep as dependency

dependencies = ["xarray_plotly~=0.0.13"]

• Simpler maintenance — changes to xarray_plotly automatically propagate
• Users who use xarray_plotly directly get a consistent version
• Risk: breaking changes in xarray_plotly affect fluxopt-plot

Option B: Inline (vendor)

• Copy relevant modules into fluxopt_plot/_vendored/
• Full control over behavior
• No transitive dependency risk
• More maintenance burden for updates

Recommendation

Keep as declared dependency with ~= pin. Same author, controlled release cadence. Inline only if xarray_plotly's API proves unstable.

Refs

Part of #47 — Architecture: Package Separation, SolutionResult Design, and Stability Tiers
Related: #51