Render foreground grayscale filters

[wieslawsoltes]

Unchanged Code OSS uses filter: grayscale(1) for inactive/open session visuals and Chat achievement badges, but WebScene currently paints those surfaces in their original colors. This change resolves bounded number/percentage grayscale values and applies the standard luminance interpolation matrix through the existing retained filter group in Avalonia/Uno and Flutter.

The scene struct and version remain unchanged: kind 30 reserves bit 30 for grayscale and carries its 0–1 amount in stroke_width; bit-31 brightness and zero-flag opacity behavior remain intact, with kind 31 as the balanced end. Compound filters, animation/interpolation, other functions, masks, and backdrop sampling remain under #256.

Validation debt: per the implementation-throughput priority, tests, browser/native pixel contracts, performance/lifecycle benchmarks, hosted checks, CI monitoring, and exact-package visual acceptance were skipped. git diff --check passes on source head 73498726 rebased onto main 6363114a.

Closes #468
Parent: #256