refactor: extract self-contained colormaps module

[patrickoleary]

refactor: extract self-contained colormaps module

MIGHT BE NICE to extract to trame-colormaps

Extract all colormap logic from view_manager.py and utils/ into a self-contained colormaps/ module with clean layer separation.

Core (pure ParaView/VTK/numpy — zero Trame dependency)

• core/presets.py: preset discovery, COLORBAR_CACHE, lut_to_img()
• core/ticks.py: tick computation for linear, log, symlog scales
• core/transforms.py: LUT transforms (linear, log, symlog, discrete)

Server / State / Controller

• server.py: ColormapInitialize() — one-time Trame server setup
• state.py: ColormapConfig reactive state model with full field docs
• controller.py: ColormapController — owns LUT, wires mapper, manages presets/range/ticks

Widgets

• widgets/colorbar.py: colorbar strip with tick overlay and range labels
• widgets/control_panel.py: preset picker, scale, range, discrete settings

Documentation

• README.md: public API, dependencies, numpy justification, module structure, layer separation, widget DOM tree, config fields, usage patterns