add a paletteLerp equivalent for p5.strands

[Nixxx19]

in regular p5, paletteLerp lets you blend across multiple color stops at given positions, which is super handy for gradients. there is no equivalent inside p5.strands shaders right now, so anyone wanting a multi stop gradient on the gpu has to chain mix() calls or write their own helper.

would be cool to have something with similar ergonomics that works inside strands callbacks. exact api is up for discussion since strands has its own way of expressing things, but the goal would be feature parity with the cpu paletteLerp.

came out of the same call. opening this so it doesn't get lost.

[LalitNarayanYadav]

I’d like to have a go at it.

My proposed approach:

• Add overloaded GLSL functions in strands_builtins.js for 2–8 color stops (GLSL ES 3.0 has no variadic args, so typed-array overloads by size is the idiomatic solution)
• Extend the transpiler to rewrite [color(…), …] array literals → vec4[N](…) constructors so the call site mirrors the CPU API exactly:

paletteLerp(
  [color(255, 0, 0), color(0, 255, 0), color(0, 0, 255)],
  [0.0, 0.5, 1.0],
  uv.x
)

Does this direction sound right ? Also, should the WGSL builtin equivalent be in scope here or a separate issue?