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.)

lib/io/io.ww

@@ -6,12 +6,12 @@
 // tagged unions instead of errno-style integer sentinels.
 //
 // This file owns the eof / underread variant tags; lib/io/stream.ww
-// owns the `vtable` + `stream` + read/write/close dispatchers, and
-// lib/io/types.ww owns the error union, mode/whence enums, and the
-// reader/writer/closer fn-type aliases. #94 fold-eFinal collapsed the
-// pre-vtable `stream` struct + `closed` tag into the single vtable
-// surface; the dispatchers are read/write/close (over `stream`),
-// final over `handle` at io fold-2 (#5).
+// owns the `vtable` + `stream` + read/write/close dispatchers and the
+// [[empty]] singleton, and lib/io/types.ww owns the error union,
+// mode/whence enums, and the reader/writer/closer fn-type aliases.
+// #94 fold-eFinal collapsed the pre-vtable `stream` struct + `closed`
+// tag into the single vtable surface; the dispatchers are read/write/
+// close (over `stream`), final over `handle` at io fold-2 (#5).
 package io;
 
 // eof — read past the end of the stream. Hare uses the `done`