This PR makes 2 big changes that will affect developers:

1. Qt5 has been removed from the project - Everything is now compiled and tested against Qt6
2. Out-of-source build - when cmake was introduced it was overlaid on the existing build-in-source structure. This means that it's very easy to contaminate the repo when building and developing. This PR moves to out-of-source building (which is the recommended best-practice with cmake). New installation instructions are provided in the INSTALL.md document.

Now for the details:

• Dropped Qt5 CI path and standardized builds on Qt6: removed the Qt5 GitHub Actions job (packages, config flags) and simplified the remaining Qt6 job to the default build directory and standard config. Closes issue Drop all project support for Qt5 as it's EoS #967
• Tightened build hygiene: enforced out-of-source builds (allowing only build and build-*), unified cmake_minimum_required to 3.16 across root, tools, and prototypes.
• Repo hygiene: added *.tmp to .gitignore and removed stray tlist.tmp.
• Docs refreshed: expanded BUILD/INSTALL/CONTRIBUTING/README with clearer quick-starts, install/staging instructions, and troubleshooting aligned to the new build expectations. These docs probably need some additional proofing.