Render Code OSS foreground brightness filters #463

Parent: #256

Problem

Unchanged Code OSS uses single-function foreground brightness filters in release notes, the issue reporter, Chat status/debug surfaces, and dictation visuals. WebScene retains the computed filter value but publishes no retained filter group, so these surfaces paint at their unfiltered colors.

Focused implementation

• Parse a single brightness(<number-or-percentage>) function with a bounded 0–10 multiplier.
• Wrap the element and descendant paint in the existing balanced kind 30/31 retained group.
• Reserve kind-30 flags bit 31 for brightness and carry the multiplier in stroke_width; preserve zero-flag opacity groups.
• Apply the equivalent RGB color matrix in the shared Avalonia/Uno Skia presenter and Flutter.
• Treat non-none foreground filters as atomic stacking contexts.

Compound filters, filter animation/interpolation, blur and expanded damage, other filter functions, masks, backdrop sampling, and isolation remain later #256 slices.

Validation debt

Implementation throughput is the current priority. Tests, browser/native pixel contracts, performance and lifecycle benchmarks, hosted checks, CI monitoring, and exact-package visual acceptance are explicitly deferred. No broader support or release claim is made from this implementation alone.