[geom] Fix stale thread-local navigators across manager lifetimes

[agheata]

This Pull request:

Repeatedly executing a geometry macro that creates a TGeoManager, calls
CheckOverlaps(), and draws the geometry can crash when interacting with the
canvas.

The parallel overlap check creates navigators for its worker threads.
TGeoManager::GetCurrentNavigator() caches these pointers in thread-local
storage. Deleting the manager destroys the navigators, but the caches belonging
to persistent worker threads remain alive. When these workers are reused for a
new geometry, they can return a dangling navigator from the previous manager.

Changes or fixes:

• Associate the thread-local navigator and thread ordinal with the current
  manager and an atomic geometry-state generation.
• Invalidate cached state when navigator or thread state is destroyed or reset,
  including during manager teardown.
• Update only the calling thread's cache when adding or selecting a navigator,
  preserving independent navigators created concurrently by different threads.
• Synchronize shared navigator-map lookups and first-time thread registration.
• Replace the manager-blind thread_local initialization flags in the parallel
  overlap checker with a check against the current manager.
• Add gtest-geom-manager-lifetime, covering:
  • concurrent creation and selection of independent per-thread navigators;
  • repeated manager recreation after a parallel CheckOverlaps() call.

There are no public API, class-layout, dictionary, or I/O changes.

Checklist:

• tested changes locally
• updated the docs (if necessary)

This PR fixes the issue reported in the forum: https://root-forum.cern.ch/t/crash-in-geometry-after-checkoverlaps/64954

[github-actions]

Test Results

    23 files      23 suites   3d 14h 24m 10s ⏱️
 3 878 tests  3 877 ✅ 0 💤 1 ❌
78 979 runs  78 978 ✅ 0 💤 1 ❌

For more details on these failures, see this check.

Results for commit d6d85f1.

[dpiparo]

Thanks. Please merge at your earliest convenience and backport to 6.40.