S3 (perf plan_3 §4.6): number of slot-eligible locals this block frame
declares directly — the size of the per-frame _slots array the
interpreter pre-allocates on entry. Populated by StaticResolver (a
block-local is slot-eligible iff every occurrence is a depth-0,
non-escaped, non-assignment access; ineligible locals stay name-based in
_values and are not counted). null = not yet resolved → no slot array.
Mutable because the resolver assigns it on block exit. Emitted into
toJson only when non-null, so an unresolved block serializes identically
to the pre-S3 format.
