S2 (perf plan_3 §4.3 step c): statically-resolved lexical coordinate of
this identifier use — the number of enclosing Environment hops to the
declaring scope (resolvedDepth) and the declaration index within that
scope (resolvedSlot). Populated by the static scope resolver pre-pass
(StaticResolver) and serialized, so the analyzer-free Flutter precompute
path interprets with zero resolution cost. null = unresolved → the
interpreter falls back to the name-keyed Environment.get path. Mutable
because the resolver assigns them after construction. In S2 these are
carried + validated by an assert only; S3 routes reads onto them.
