Bivariate Support

[TheJeran]

New Features

• Bivariate colormapping — Visualize two variables at once using a 2D color scheme (blended from four corner colors: bottom-left, top-left, bottom-right, and a configurable mix mode/resolution), instead of the previous single-variable colormap.
• Second variable selection — New SecondVariable / VariableCombo UI lets users pick a second variable to pair with the primary one, with a quick "swap" control to flip which variable maps to which axis.
• Bivariate colorbar — A new 2D colorbar (BivariateColorbar, BivariateColor, BivariateCanvas) with axis labels, hover support, and titles, alongside the existing univariate colorbar (UnivariateColor, UnivariateColorbar).
• Point cloud bivariate coloring — Bivariate color mapping now also works in point cloud rendering mode, in addition to flat, sphere, and their displacement/volumetric variants.
• Bivariate-aware image export — Export now supports multiple colorbar labels/units (cbarLabels/cbarUnits arrays) so exported images correctly reflect both variables.

Refactors / Breaking Internal Changes

• Global store now supports arrays for per-variable state: valueScales, units, scalingFactor → scalingFactors, and colormap-related state moved out of GlobalStore into a new dedicated ColormapStore.
• variable is now string | undefined (was defaulting to "Default"), with a new variable2: string | undefined for the second variable, and a new bivariate: boolean / shareScale: boolean flag pair.
• zMeta is now object[] | undefined instead of defaulting to [{}].
• Shader chunks reorganized: new shared commonHelpers.glsl / commonUniforms.glsl, plus blend-mode support added to fragment shaders (flatFrag, sphereFrag, sphereBlocksFrag, pointFrag, rayMarchFragment, DDAFrag).

Fixes / Stability

• Multiple "code review" passes fixing shader/UI stability issues introduced during bivariate development (per commit history: "stability changes from code review," "Code review bug fixes and shader tweaks").
• Adjustment menu settings now conditionally hidden depending on univariate/bivariate mode.

Removed

• TextureStore.ts deleted — its responsibilities were absorbed into ColormapStore/GlobalStore.