Task 1-2 of the bound-override cycle: PrimitiveBuilder::bind/try_bind stop
capturing the bound value in a wrapped build closure and store it only as
BoundParam data; build() merges bound values back into the full-arity vector
at call time (ascending original position — behaviour-identical to the
retired closure chain). This makes the reversal possible: unbind(slot)
returns the param to the declared surface at its original order.
aura-engine gains the path-addressed Composite::reopen (mirroring
collect_params' prefix rules, lockstep with expansion_map) plus the
read-only bound_param_space() enumeration (path-qualified BoundSpec with
values) and the ReopenError/BoundSpec exports. An e2e proves a reopened
param rebuilds from the freshly supplied value through compile_with_params,
not the stale default.
refs #246
