wcc: stamp for-range tuple-binds and massign blank via shared helper

For-range tuple-destructure binders (for (let (k,v) .. s)) and the tuple
massign discard _ were left nil-typed: the for-range binders are N_IDENT
use-sites and the _ slot, though unbound, has a real element type. Add a
shared stamptuplebinds helper — one lockstep walk distributing an N_TTUPLE's
per-element types onto a binder chain — refactoring the existing N_MLET
destructure loop into it (behavior identical) and adding N_FORRANGE and
N_MASSIGN call-sites. _ is STAMPED with its slot's element type (unbound is
not untyped), not exempted. Mirrors harec create_unpack_bindings
(ref/harec/src/check.c:1354-1419), the routine harec shares between let-unpack
and the for-each header (:2308-2317).

A prerequisite for arming the wwstage asserttyped bail. byte-id holds (cgen
derives binder/elem widths structurally, never off type_; 990-997 green).
Extends the 901 gap-corpus with 901_forrange_tuple.ww + 901_massign_blank.ww.

test/wcc/901_massign_blank.ww

@@ -0,0 +1,25 @@
+// 901_massign_blank — tuple multi-assign discard `_` gap fixture for the
+// 901 asserttyped net (class G). In `_, a = f()` the blank `_` parses to
+// an empty-str N_IDENT with no decl, so the generic N_IDENT exprtype path
+// left it type_=nil (945's own comment documents the gap). The `_` is a
+// GAP, not an exemption — the slot has a real element type ([]u8 here), so
+// stamptuplebinds(define=false) stamps it with that element type (harec
+// drops `_` but still advances the tuple slot). a is pre-declared and
+// resolves the normal way; this reads 0.
+package main;
+
+fn mk() ([]u8, i64) = {
+	let b: str = "hi";
+	let p: []u8;
+	p.ptr = b.ptr;
+	p.len = 2;
+	p.cap = 5;
+	return (p, 7i64);
+};
+
+export fn main() i32 = {
+	let a: i64 = 0i64;
+	_, a = mk();
+	if (a != 7) { return 1; };
+	return 0;
+};