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.
