Render foreground brightness filters

[wieslawsoltes]

Unchanged Code OSS uses single-function brightness(...) filters in release notes, the issue reporter, Chat status/debug surfaces, and dictation visuals, but WebScene currently publishes their unfiltered colors. This change resolves bounded number/percentage brightness values, wraps the element and descendant paint in the existing retained group scope, and applies the corresponding RGB color matrix in Avalonia/Uno and Flutter.

The scene struct and version remain unchanged: kind 30 reserves its top flag bit for brightness and carries the multiplier in stroke_width; zero-flag opacity groups keep their prior alpha behavior and kind 31 remains the balanced end. Non-none foreground filters now establish an atomic stacking context. Compound filters, filter animation, blur damage, 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 88a82009 rebased onto main c19fbc36.

Closes #463
Parent: #256