Nested (2-level) variant of misc_global_defer. Each deferring Worker lives in its
own sub-system `Sub`; `Top` composes two Subs (sa, sb) under one Main.

SCOPE RESULT: dzn's global defer FIFO spans ACROSS the two sub-systems.
  Top_golden.aut (`dzn graph -b lts -f aut -m Top`) is BYTE-IDENTICAL to the flat
  misc_global_defer/System_golden.aut: strictly m.aDone then m.bDone (11 states,
  14 transitions). The global queue does NOT reset at sub-system boundaries.

ours (`nucleos lts -m Top`): 44 states / 48 transitions, both orders reachable.
ltscompare (-eweak-trace, m.inevitable->tau): not equal; dzn ⊑ ours (true);
ours ⊑ dzn (FALSE) — same strict-superset over-approximation as the flat case.

IMPLICATION for the fix: the layered binary-product monitor must CARRY the defer
events (<defer-qin>/<defer>/<defer-cancel>) all the way up — one system-wide FIFO
for the whole flattened composition, regardless of nesting depth. The queue cannot
be hidden/reduced at an intermediate sub-system boundary.

GOLDEN NOTE: *_golden.aut is generated via scripts/lbs-system-golden.sh (inspect-ports + dzn graph + lbs_system_transform.py), which strips dzn's known provides-port .inevitable BUG (+ tau self-loops, fork-at-reply). Validate with: NUC_DEFER_SYNC=1 NUC_RTCPROD=1 nucleos lts -m <C> | wf-check.py aut - <golden> --system.
