feat: image mode obstacle driven by luminance (obstacleStrength prop)

[jayf0x]

Goal

Currently the image obstacle texture is created as brightness(effect) blur(8px) where effect defaults to 0 → brightness(0) = solid black = no obstacle. Most <FluidImage> users never see obstacle behaviour.

Proposed API

```tsx
// new prop: 0 = no obstacle (current default), 1 = full luminance-based obstacle

```

Bright image regions mildly deflect fluid at 0.5, creating natural interaction between image content and simulation.

Acceptance criteria

• obstacleStrength prop added to FluidBaseProps / globals.d.ts
• Prop controls how much image luminance contributes to the physics obstacle
• Default 0 — identical to current behaviour
• obstacleStrength={1} produces visible fluid deflection at bright image regions
• Tests updated

Files likely touched: src/globals.d.ts, src/core/textures.ts, src/core/config.ts