Image export: choose screen-matched vs export-fit color mapping (palette window + equalization), with a preview in the save modal

[rosslh]

Context

The PNG export makes opinionated choices about how its color mapping is derived, with no user control:

• Equalization table: with histogram coloring active, the export equalizes against its own escape-value distribution (export-fit, like the zoom animator's frames). Before c85de94 it inherited the map's viewport-global table (screen-matched), which miscolored aspect-expanded exports — but always matched what the user saw on screen.
• Palette window: the export keeps the config's window — under auto-fit, the viewport's fitted min/max — even though the exported region (aspect-adjusted bounds) and resolution differ from the screen. A fully export-fit mapping would refit the window to the export's own values when auto-fit is on.

So the current behavior is a hybrid: screen-matched window, export-fit equalization. Both pure mappings are legitimate, and neither is right for everyone:

• Screen-matched (viewport window + viewport table): the saved PNG reproduces exactly the view the user judged and decided to save — but content added by aspect-ratio bounds expansion is colored through a mapping it never entered.
• Export-fit (window refit to the export under auto-fit + table fit to the export): the export is colored coherently as the image it is — but can differ from what was on screen at save time.

Proposal

Support both, as an option in the save-image modal, with a small live preview so the difference is visible before committing to a full-resolution render:

• A control in saveImageModal choosing the mapping. Each choice applies as a coherent bundle (window + equalization together), not as independent toggles.
• A preview thumbnail rendered at the requested aspect ratio through the worker pool (e.g. ~300 px wide via RegionRenderer.renderRegion + recolor), re-rendered when the mapping choice or dimensions change. The preview also incidentally shows the aspect-ratio bounds expansion, which is otherwise invisible until the file is saved.

Implementation notes

• Screen-matched = config window + the map's paletteCdf (see MandelbrotMap.rebuildPaletteCdf).
• Export-fit = fittedRangeForRender-style window refit over the stitched export values + fittedCdfForValues (see ImageSaver.exportCdf); ImageSaver.generateImageColumns already renders columns for values and recolors through a shared table, so the window refit slots into the same pass.
• PNG/data-export metadata should keep pinning the window actually used (pinnedConfig), whichever mapping produced it.

Non-goals

• The zoom animator's analogous choices (relative vs absolute manual window across depths, per-frame vs locked-to-target grading): previewing a whole zoom's coloring is expensive, and the animation flow deserves its own design if ever.
• Making aspect-ratio handling configurable (expand vs center-crop): a geometry decision, separate axis from color mapping; the preview at least makes the current expansion behavior visible.