Fix fill opacity saturation from duplicate segmentation layers

[projectsahmaddurrerehman-spec]

Repeated segmentation renders during a slow NIfTI load can each create a new labelmap actor before the first actor is ready. The layers then compound their opacity: four layers at 50% produce approximately 94% coverage, making the upper half of the Fill slider look unchanged.

Attach one actor with a stable ID before registering the segmentation representation. Apply the same loading sequence to the single-case viewer, comparison viewer, and HD representation rebuilds. Keep the existing fill/outline controls and the zero-opacity fix.

Validation:

• Regression using Cornerstone's real labelmap renderer: failed before the change with four actors instead of one; passes after the fix.
• Seven targeted tests pass, covering slow loading, repeat registration, HD actor replacement, load failure, 0%, 50-100%, hidden labels, inactive labels, and independent borders.
• Full local frontend suite: 345 tests passed in 55 files.
• TypeScript checking and production build passed locally.
• All GitHub checks passed, including frontend tests/builds on Node 20 and 22, backend security tests, secret scan, dependency audit, and CodeQL.
• Deployed to JHU at main commit 5353292. The server production build passed; home, dashboard, viewer, and comparison routes serve the new build; public entry/viewer JavaScript matches the built files byte-for-byte; search and backend health checks pass; CT and labelmap range requests return valid gzip data (HTTP 206). Previous frontend assets are backed up for rollback.
• Live Chrome slider verification remains pending: the required built-in browser tool fails to load its request-header policy even after a runtime reset. No alternative browser automation was used.