Add memoization to Color parsing and serialization methods

[obiot]

Description

The Color class methods for parsing and serializing color values could benefit from memoization to avoid redundant computation:

Parsing (cache by input):

• parseRGB() — parses "rgb(...)" / "rgba(...)" strings
• parseHex() — parses "#RRGGBB" / "#RRGGBBAA" hex strings

Serialization (cache by r,g,b,a state, invalidate on change):

• toHex() — returns "#RRGGBB"
• toHex8() — returns "#RRGGBBAA"
• toRGB() — returns "rgb(R,G,B)"
• toRGBA() — returns "rgba(R,G,B,A)"

For serialization, the cache should be invalidated whenever r, g, b, or a values change (e.g. via setColor, parseHex, etc.).

Context

These were tracked as inline TODO comments in src/math/color.ts. Moved to a ticket for better visibility and tracking.