Brummel
db710b1a73
A `let`-bound `loop` whose body references one of its own loop binders on the non-recur exit passed `ail check` but failed `ail build` with `unknown variable`. The fieldtest surfaced this with an owned RawBuf loop binder, but the trigger is element-type-independent. Root cause: the Term::Loop arm of synth_with_extras (the type-replay walk the let-lowering runs via synth_arg_type on every let value) descended into the loop body WITHOUT adding the loop binders to `extras`, unlike the Term::Let arm which pushes its binder. A loop binder referenced on the non-recur exit then resolved to UnknownVar during the type replay. Fix: clone `extras`, push each loop binder's (name, ty), and thread the augmented extras into the recursive synth of the body — mirroring the Term::Let arm exactly. General fix; a plain Int let-bound loop that references its binder also builds now. This restores check<->codegen agreement for the natural fill-loop (thread an owned buffer of runtime length through a loop/recur, one RawBuf.set per iteration) — the way to populate a buffer whose length is not a fixed set of literal indices. Surfaced by the raw-buf fieldtest (finding B2, docs/specs/0058). RED-first: loop_let_bound_binder_reference_builds.