feat: add reusable custom post-process view

[nmfisher]

Summary

• add a reusable fullscreen custom post-process view backed by sampleable HDR color and depth targets
• support redirecting an existing view while preserving Flutter/platform render targets
• bind the conventional tDiffuse, tDepth, and texelSize material parameters
• support resize and deterministic resource cleanup
• export CustomPostProcess through the public thermion_dart API

This is the core rendering primitive used by the Sakura explorer for its live depth-ink, color-grade, vignette, and FXAA finale. The API accepts public FilamentApp, View, and MaterialInstance types; callers no longer need to import the implementation file directly.

Validation

• scoped dart analyze: clean
• package-wide dart analyze: no new diagnostics (49 pre-existing test/tool warnings)
• vertex_index_buffer_tests: could not initialize Filament in this environment because no X display is available; timed out during FFIFilamentApp.create

Scope

Current develop already contains the render-thread shadow mutations and newer BufferObject APIs that originated on the Sakura branch, so those changes are intentionally not duplicated here.

[nmfisher]

Closing in favor of keeping this shader-specific post-process controller in the Sakura package. We will use composition around Thermion View/RenderTarget primitives and submit only any genuinely missing public engine APIs separately.