feat: RMSE/R² distribution plots over valid fits with fit selection

[ahmadomira]

Summary

Add RMSE/R² distribution plots over all accepted fits (alongside the existing parameter distributions), plus a UI to inspect and select another valid fit to apply/overlay. This requires exposing per-accepted-fit RMSE/R² from the pipeline — currently only parameter samples are retained on FitResult.

Source

Reported via the fitting-app feedback channel.

Acceptance criteria

• RMSE/R² distributions of accepted fits are shown in the distributions view.
• The user can select an alternate valid fit, and the selection updates the displayed result/plot.
• The pipeline exposes accepted-fit RMSE/R² samples on the result.
• Tests for the new pipeline data and the selection behaviour; uv run ruff check / format clean.

Scope & file boundaries

Package F. Touch:

• gui/plotting/distribution_widget.py
• gui/plotting/plot_widget.py
• gui/fitting_session.py
• core/pipeline/fit_pipeline.py

Parallelization

Package F — start only after Package E (average-dots) PR has merged; branch from the updated main.