fix(compositor): high-quality motion blur (cursor & screen) and gaussian webcam blur

[EtienneLescot]

Context & Problem Description

Users reported severe quality degradation when using motion blur and webcam background blur:

1. Cursor Motion Blur: When cursor motion blur is enabled, moving the cursor produces a trail of disconnected ghost replicas (multi-exposure stepped ghosts, e.g. 👆 👆 👆 👆) rather than a smooth directional smear. Additionally, the active cursor head becomes faint and washed out.
2. General Screen / Camera Motion Blur: The \effects.motionBlur\ slider only scales the number of taps (\mb_taps\ 1..16) but does not scale the displacement vector (\duv), which always spans 100% of 1 full frame. Consequently, at low slider settings (e.g. 0.2), the shader samples a large 1-frame movement with only 4 discrete taps, creating visible quadruple ghost images.
3. Webcam Background Blur (Segmentation): The \�lur_webcam_bg\ shader function uses a 5x5 Cartesian grid with up to 14px step size and an inverse-distance weight (\1 / (1 + r)). This results in harsh grid/cross artifacts, dithering, and 25 discrete pinhole dots on point lights instead of a soft Gaussian bokeh.

Root Cause Analysis

• Cursor:
  • \ rail_frames\ in \rame_geometry.rs\ and \compositor_windows.rs\ is set to \1.0 + blur01 * 7.0\ (spanning up to 8 frames / ~266ms into the past).
  • \ aps\ is capped at 11, resulting in 10-30px gaps between discrete cursor quads.
  • Uniform tap weighting (\1 / taps) reduces current cursor opacity to ~9%.
  • \compositor_windows.rs\ duplicates \plan_cursor\ inline rather than sharing \rame_geometry::plan_cursor.
• General Motion Blur:
  • \mb_taps = 1.0 + motion_blur * 15.0, but \duv = uv_now - uv_prev\ is always full frame displacement. The slider acts as sample count rather than shutter angle/duration.
• Webcam Background Blur:
  • 25 unrolled taps on a coarse grid with large spacing and non-Gaussian falloff.

Objectives & Requirements

1. Cursor Motion Blur:
   • Limit temporal window to single-frame shutter duration (\shutter_dt <= 1.0 / FPS).
   • Adaptive or sufficiently dense sampling so consecutive taps overlap continuously.
   • Front-weighting (falloff from current cursor to past) so the cursor head remains sharp and visible.
   • Unify Windows, macOS, and Linux backends via \rame_geometry::plan_cursor.
2. General Motion Blur:
   • Scale blur vector by \effects.motionBlur\ (shutter angle).
   • Trailing sampling ending at \uv_now.
   • Smooth reconstruction without ghosting at low slider values.
3. Webcam Segmentation Blur:
   • Replace 5x5 Cartesian grid with true Gaussian disc/spiral sampling (Vogel spiral / multi-ring) with Gaussian weights (e^{-r^2 / 2\sigma^2}) and high-frequency dither (Interleaved Gradient Noise).
   • Keep tap budget <= 20-24 taps for maximum efficiency on integrated GPUs.

[EtienneLescot]

Implementation & Resolution Summary

We have addressed all points raised in this issue with cross-platform shader and compositor updates:

1. Cursor Motion Blur Quality & Phantoms:

   • Restricted shutter interval from the previous 8 frames (up to 266ms) down to the current frame window: \ rail_dt = blur01 * (1.25 / FPS).
   • Adaptive tap density: stationary cursors drop to 1 tap (no overhead), while moving cursors scale dynamically with displacement (\clamp(2, 16)).
   • Front-weighted linear tap ramp: the leading cursor head retains 100% full weight, decaying smoothly to 25% at the tail with normalized unit opacity sum (\cursor_tap_weight).
   • Blended per-tap in D3D11, Metal, and Vulkan backends.

2. General Motion Blur (Zoom & Pan):

   • Passed \mb_amount\ in constant buffer (\LayerCB.mb.y).
   • Scaled blur trajectory by \saturate(mb.y)\ backwards from current frame \uv_now. Below 100% blur, taps no longer stretch across the full inter-frame displacement, eliminating ghosting.
   • Early exit when \mb_scale <= 0.001\ or displacement is negligible.

3. Webcam Segmentation Background Blur (Gaussian Bokeh):

   • Replaced the 25-tap Cartesian grid with an isotropic 21-tap Vogel spiral (golden angle disk) Gaussian bokeh kernel.
   • Applied per-pixel rotation using Interleaved Gradient Noise (IGN).
   • Results in smooth photographic circular bokeh with 16% fewer texture samples (21 vs 25 taps).

All 168 Rust unit/geometry tests passed (including the pinned frame geometry regression test and shader entry point compilation tests), native release binaries were rebuilt, and TypeScript/linter checks are completely green.