resolvewalk N_MLET arm distributes the N_IDENT-callee rhs return-tuple
element types onto unannotated bindings (the A-narrow slice). Byte-id-
neutral — cgen still classifies structurally, stamps inert until the
exprfloatkind collapse. N_DOT-callee destructure deferred to #16/#17.
Prereq for the #121 collapse (commits 2/3).
The fix's new cglet branch is gated on sz==16 AND rettupleof != nil. The
existing rows prove it CATCHES 16B tuple receives but nothing pinned that
it does NOT catch a 16B struct{i64,i64} receive (same sz==16, but
rettupleof returns nil for a non-tuple return). A future rettupleof
refactor returning non-nil for a struct would silently route the struct
through the AX/DX tuple spill and miscompile it, gate-blind. The new
ctl_struct row is byte-id and runs correctly (exit 12) both pre- and
post-fix, pinning the tuple/struct discrimination.
Per-row: cstage build+run exit assert AND w6c vs w6c_ww .s cmp.
Three scalar-pair single-var bug-rows — (f64,i64), (i64,f64) order-swap,
(i64,i64) all-integer — each diverge on master 60c3e51 by one dropped
MOVQ DX and are byte-identical after the fix. The all-integer row proves
the fix is not f64-gated. Two control rows stay byte-id pre- and
post-fix: the destructure form `let (a,b) = mk()` (cgmlet + tupstore
cursor, the path bootstrap/990-997 rely on) and a 32B str-element
single-var tuple (excluded by the fix's sz==16 gate). Modelled on
953_f64crossmod_run. Master gate: 3 bug rows FAIL, 2 controls clean.
