Render compound foreground filter lists

[wieslawsoltes]

Problem

WebScene's retained emitter only recognized exact single foreground filter functions. Valid compound lists were omitted, authored order could not be represented, and Code OSS's blur-plus-saturation surfaces remained incomplete.

Change

• parse up to 16 whitespace-separated foreground functions as one all-or-nothing list
• support repeated brightness, grayscale, contrast, saturation, and pixel blur operations
• add matching Skia/Flutter saturation matrices
• open saved layers in reverse-authored nesting order so restoration applies CSS functions left-to-right
• retain balanced groups across normal and early SVG scene paths
• add native retained-order and browser computed-value regression coverage

Unsupported functions and unresolved functional lengths remain fail-closed and tracked by #256.

Validation

Tests and builds were deliberately not executed under the current implementation-throughput directive. Browser/native pixels, effect damage, performance, memory, lifecycle, package, and CI gates remain required before release promotion and are recorded in #487/#256/#235.

Closes #487