Follow-up tracker: PR #2463 cmake/Boost/Qt modernization

[dcwhite]

Context

PR #2463 (cmake/Boost/Qt/Python modernization) spun off several follow-up issues during review rather than expanding scope in-PR. Tracking them here.

Checklist

• Debug-build test failures masked by Eigen assertions (mac-coverage job) #2546 — Debug-build test failures masked by Eigen assertions (mac-coverage job)
• Upgrade Tetgen to 1.6 and test it #2547 — Upgrade Tetgen to 1.6 and test it
• Upgrade Boost to 1.91 #2548 — Upgrade Boost to 1.91
• Clean up Qt6 Windows main-window-flicker fix (#2484) #2549 — Clean up Qt6 Windows main-window-flicker fix (Qt6 Windows build: entire SCIRun app disappears momentarily when you add a saved snippet #2484)