more algorithm-independent GC infra - #7194

separated from #6849

Summary by CodeRabbit

• New Features

  • CPython-compatible generational garbage collector with cycle detection, finalization, resurrection handling, and uncollectable tracking.
  • Deferred-drop and trashcan mechanisms to safely handle deep or recursive deallocation.
  • Public GC triggers to run targeted collections.

• Bug Fixes

  • Reordered interpreter shutdown to run atexit, set finalizing, force GC, then finalize modules—improves finalization and unraisable reporting.
  • Improved weakref/finalizer and GC untracking semantics.

• Chores

  • Spell-check vocabulary expanded; GC feature enabled in build config.