lib/io: empty() stream; wcc/cgen: #129 sretretsize + #130 global tagged-field store

io.empty (discard+EOF stream, ref/hare/io/empty.ha:4-17) — needed by getopt's
two-pass printusage width measurement. Diverges from Hare's `const empty: *stream`:
a `let _empty_vt` + `fn empty()` that wires the fn-ptr slots per call, because
const-init of a vtable struct with fn-ptr fields is blocked (#118, ruled accept).

Co-discovered while making empty() byte-identical across stages: three
wwstage-only cgen fixes (cstage was already correct; wwstage aligned down):
- #129 sretretsize: consult the same-module pointer-alias before structlookup's
  any-module struct fallback (io.stream = *vtable was mis-sized as memio's 56B
  struct -> spurious sret save).
- #129 callsretsize: swap curmod to the callee's module before sret-size
  classification (cross-module callee context).
- #130 cgassign global-struct tagged-union field store: add the missing arm
  (was a 1-word store) mirroring cstage cgen.c:4893-4912.

The three are inseparable from io.empty here — splitting them out leaves a
divergent-asm intermediate (993/995 red), so they ride one commit per the
one-class gate-repair carve-out (#133-expanded precedent). Regenerates the
embedded combined.ww; 989_lib_byteid pins bufio + fmt graduated to M_ID.
(cgenexpr.ww fix-3 inline comment cites the #129 cluster; narrow to #130 on
next touch to avoid a regen for a comment.)

selfhost/cmd/wcc/cgenexpr.ww

@@ -9891,6 +9891,23 @@ fn cgassign(c: *cgen, n: *node) void = {
 											return;
 										};};
 									};
+									// #129: tagged-union field on global struct. LEAQ
+									// base(SB) into BX then the shared widener handles
+									// every rhs shape. Mirrors cstage cgen.c:4902
+									// is_global arm. Without this the generic TK_ASSIGN
+									// below truncates to 1 word, silently dropping tag
+									// and payload.
+									if (n.op == tkind.TK_ASSIGN
+									    && istaggedtype(c, fi.tnode)) {
+										let fsz: i32 = slotsize(c, fi.tnode);
+										emitline("\tLEAQ\t");
+										emitsymname(c, bn);
+										emitline("(SB), BX\n");
+										cgwidentaggedstore(c,
+										    fi.tnode.type_: *tinfo,
+										    n.rhs, "BX", fi.foff, fsz);
+										return;
+									};
 									if (n.op == tkind.TK_ASSIGN) {
 										cgexpr(c, n.rhs);
 										if (isstrtype(c, fi.tnode)) {