feat(shadertoy): add proper mouse/touch interaction support

[ochafik]

Summary

This PR fixes and enhances mouse interaction in the Shadertoy example to match the official Shadertoy specification.

Changes

iMouse Uniform Behavior (Spec-compliant)

Component	When Button Down	After Release	Never Clicked
iMouse.xy	Current position	Last position	(0, 0)
iMouse.zw	Click start (positive)	Negated (-x, -y)	(0, 0)

Previous behavior (incorrect):

• iMouse.xy always updated on mousemove (should only update when button down)
• iMouse.zw reset to 0 on mouseup (should negate to indicate release)

New behavior (spec-compliant):

• iMouse.xy only updates while button/touch is held
• iMouse.zw is positive when down, negative when released
• iMouse.z > 0 reliably indicates button is pressed

Mobile Touch Support

Added touch event handlers for mobile devices:

• touchstart / touchmove / touchend / touchcancel
• Same behavior as mouse events with proper coordinate translation

Documentation

• Updated tool description with mouse interaction guide
• Enhanced README with iMouse behavior table and examples
• Fixed Interactive Julia Set example to handle initial state

Research

iMouse is a core Shadertoy feature expected by the community. Most popular shaders use it:

• Star Nest (camera rotation)
• Raymarching templates (camera control)
• Particle systems (attraction/repulsion)
• Interactive fractals (parameter control)

All major Shadertoy clones/viewers implement mouse support:

• VS Code shader-toy extension
• shadertoy-react
• desktop-shadertoy
• pygfx/shadertoy

Testing

• Built successfully
• Unit tests pass
• E2E tests have unrelated browser launch issues (pre-existing)

Fixes mouse interaction to match official Shadertoy specification.