docs(release): v1.21.0rc3 notes

[dustenhubbard]

Release notes for v1.21.0rc3 (Pre-release channel), cut off fork/main 0cb078c. Adds the WHATS_NEW.md [1.21.0rc3] section and a mirrored CHANGELOG.md entry covering everything merged since rc2.

User-facing highlights

• Isolate objects/traces: Hide Other Objects, Hide all objects, Show all objects, and Invert selection (feat: isolate objects/traces (invert selection + hide others) #51)
• Readable, colorblind-safe colors for imported auto-segmentations (feat: readable colorblind-safe colors for autoseg import #50)
• The 3D scene now tracks live 2D trace edits (fix(3d): refresh stale scene meshes after 2D trace edits #49)
• Copy-to-sections and correlation-propagation hardening (fix(copy-to): guard section-range input and correct cross-section trace placement #46, fix(align): propagate respects locked sections and correct corrAlign composition #47)

Docs-only; no code changes. After merge: tag v1.21.0rc3 and publish to the Pre-release channel.