Feat(canvas): add transparency locking support for gradients

[DustyShoe]

Summary

This PR adds transparency lock support to the canvas Gradient tool for raster layers, so gradients now match Brush behavior when a layer is transparency locked.

2026-05-23.09-37-53.mp4

When the active raster layer has Transparency Lock enabled, both Linear and Radial gradients are applied with source-atop, which keeps the effect inside already-opaque pixels and leaves fully transparent regions untouched.

The change also:

• centralizes transparency-lock composite operation selection
• carries the composite operation through gradient buffer state and rendering
• adds focused Vitest coverage for the new behavior
• updates the Gradient Tool docs

Related Issues / Discussions

N/A

QA Instructions

1. Open the canvas and select a raster layer that has painted content plus surrounding transparent space.
2. Enable Transparency Lock on that layer.
3. Use the Gradient tool in Linear mode and drag across both painted and transparent regions.
4. Confirm the gradient only affects existing opaque pixels and does not fill transparent areas.
5. Repeat in Radial mode and confirm the same behavior.
6. Disable Clip Gradient and confirm the transparency-lock behavior still holds across the full bbox.

Validated with:

• cd invokeai/frontend/web && pnpm exec vitest run
• cd invokeai/frontend/web && pnpm build
• cd docs && pnpm build

Merge Plan

Normal merge. This is a frontend-and-docs-only change with no backend, schema, or redux slice migration requirements.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)