Two correctness fixes found during final Phase 6 code review:
F1 (CRITICAL): GC permanently disabled — staging_merge_complete bootstrapping deadlock.
sched_begin_session reset staging_merge_complete to 0 but never set it back to 1,
so sched_end_session's guard (!staging_merge_complete) always fired, preventing GC
from ever running. select-node memory leaked indefinitely across scheduling sessions.
Fix: set staging_merge_complete=1 at the end of sched_begin_session after all
staging work (sched_account_jobs etc.) completes.
F2 (HIGH): recompact_node_ids() omitted from GC protocol in sched_end_session.
The protocol in resreq.h requires gc_sweep → recompact_node_ids. Without
recompaction, sel_node_next_id grows monotonically across sessions and
select_eval_ctx_t arrays (sized by max node_id) grow without bound.
Fix: call recompact_node_ids() after gc_sweep_select_nodes.
Also adds test_staging_merge_complete_gc_guard to document the bootstrap
invariant: sched_begin_session must leave staging_merge_complete=1 so the
GC guard in sched_end_session does not fire.
Rongsong Shen
d438742f1a