fix: ensure opacity dither fully hides at alpha 0 and fully shows at alpha 1

[lucaheft]

Summary

• Fix opacity dithering so alpha = 0 produces a fully invisible surface and alpha = 1 produces a fully opaque surface, regardless of dither mode.
• Previously, dither thresholds in [0, 1] (Bayer can hit 0, blue-noise textures can hit 0 and 1, IGN noise can hit 0) caused alpha < noise to leave stray pixels visible at alpha = 0 (and could discard pixels at alpha = 1 for blue-noise).
• Adds explicit boundary early-outs in opacityDither so the contract is enforced at the source instead of relying on the noise range.
• Applied to both the GLSL and WGSL chunks (src/scene/shader-lib/glsl/chunks/standard/frag/opacity-dither.js, src/scene/shader-lib/wgsl/chunks/standard/frag/opacity-dither.js).

Before:

After:

Test plan

• Open the dithered-transparency example and confirm a material with opacity intensity 0 renders fully invisible (no scattered dots) across all three dither modes: BAYER8, BLUENOISE, IGNNOISE.
• Confirm opacity intensity 1 renders fully opaque with no discarded pixels in all three modes.
• Confirm intermediate values still dither correctly (smooth fade-in/out as intensity sweeps 0 → 1).
• Verify on both WebGL2 and WebGPU backends.

Fixes #

Checklist

• I have read the contributing guidelines
• My code follows the project's coding standards
• This PR focuses on a single change

[Copilot]

Pull request overview

This PR tightens the contract of the standard shader’s opacity dithering so the boundary cases behave deterministically across all dither sources (Bayer / blue-noise / IGN): alpha = 0 always fully discards and alpha = 1 never discards.

Changes:

• Add early-out discard when alpha <= 0.0 to prevent stray visible pixels at zero opacity.
• Add early-out return when alpha >= 1.0 to prevent any accidental discards at full opacity.
• Apply the same boundary behavior to both WGSL (WebGPU) and GLSL (WebGL) opacity dither chunks.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/scene/shader-lib/wgsl/chunks/standard/frag/opacity-dither.js	Adds explicit alpha boundary early-outs in WGSL opacityDither to enforce fully hidden/fully opaque behavior.
src/scene/shader-lib/glsl/chunks/standard/frag/opacity-dither.js	Mirrors the same alpha boundary early-outs in GLSL opacityDither for consistent behavior across backends.

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[mvaligursky]

Thanks!