Five tasks, with the ARCHITECTURE section 6.1 revision first so the code
never contradicts the single source of truth, and the reparenting kept
atomic because scope is a required keyword argument and any split would
leave an unrunnable tree.
The Codex review caught that the planned test evidence pointed at the
wrong stubs: the ones at test_backpressure.py:176-186 cover accounting-side
degradation, not the three gate paths that actually leak to callers, and
try_acquire and try_enter have no stub at all.
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