GH-126491: GC: Mark objects reachable from roots before doing cycle collection

[markshannon]

This is an updated version of #126502

It differs as follows:

• The removal of lazy dict untracking has already been merged, so isn't present
• More care is taken to ensure the consistency of the spaces
• Some adjustments to the "work to do" calculation

• Issue: Mark all objects reachable from roots as live before doing main cyclic GC pass #126491

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @markshannon for commit 698abb3 🤖

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