fix(WebGPU): pass depth-equal fragments in the translucent pass

[sedghi]

Context

The order-independent translucent pass depth-tests with depthCompare: 'greater' (reversed z) against the opaque pass depth buffer, so translucent fragments that are exactly coplanar with already-rendered opaque geometry are rejected and silently disappear. The common case is a translucent overlay on an image slice — e.g. a segmentation labelmap or fusion layer — which renders on the OpenGL backend (equal depths pass in its translucent pass) but not on WebGPU.

Found while running Cornerstone3D segmentation overlays on the WebGPU view API. Related: #3547 (independent fixes from the same effort).

Results

• Coplanar translucent geometry (labelmap/fusion overlays on slices) renders on WebGPU, matching the OpenGL backend.
• Regular translucent-behind-opaque occlusion is unchanged (greater-equal still rejects strictly farther fragments).

Changes

• WebGPU/OrderIndependentTranslucentPass: depthCompare: 'greater' → 'greater-equal'.
• New regression test: opaque slice + exactly coplanar 50%-opacity overlay; asserts the overlay blends over the slice (renders black without the fix).
• No API changes.
• Documentation and TypeScript definitions were updated to match those changes (none needed — no API changes)

PR and Code Checklist

• semantic-release commit messages
• Run npm run reformat to have correctly formatted code

Testing

WEBGPU=1 NO_WEBGL=1 npx vitest run \
  Sources/Rendering/WebGPU/OrderIndependentTranslucentPass/test/testCoplanarTranslucentDepth.js

Fails without the fix (expected 0 to be greater than 60), passes with it. Requires the WebGPU adapter flags for headless Chromium added in #3547's vitest.config.js change when running before that lands.

• This change adds or fixes unit tests
• Tested environment:
  • vtk.js: master (07c3e9a)
  • OS: macOS 15
  • Browser: Chromium (Playwright, headless) and Chrome, Apple Metal adapter

[daker]

LGTM