scan_next: gate each Plan B dst on its own debt, not the aggregate
Plan B executes each destination independently — each is a reversed Plan A
against the single start src — but its cost was evaluated only in aggregate: the
per-candidate debts were summed and the plan admitted if the sum beat
do-nothing. That let a batch of profitable singleton dedupes subsidize an
individually-catastrophic drain. The instrumentation showed it directly: one
Plan B emitted 130 dst lines sharing one aggregate debt, 129 of them 1-ref
extents (each a real space win) and one a 9912-ref extent whose own debt is
~+521k (redirecting 9912 references onto a fresh src, priced by ref-op-cost).
The group summed to -7197, so the fat drain rode along — re-canonicalizing a
near-canonical extent instead of leaving it to accumulate and retire.
Gate each candidate on its own signed space-debt at accept time: drop it unless
its per-entry debt beats the do-nothing floor (< 0). The profitable singletons
are unaffected; the high-ref drain is declined, so the fat extent stays the
canonical src, keeps absorbing, and retires at refs-max as intended. A
force-marked candidate bypasses the gate, mirroring the FullRewrite floor's
benefit-gate bypass, since force means "rewrite regardless of benefit".
This is the last of the reference-cluster cost-model defects: retirement
(refs_fetch) settles clusters, refs_created makes ref-op-cost price ref moves,
and this gate stops Plan B from subsidizing a drain past that price.
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