w6c+wwstage: route sret dest to the global symbol on struct-return into a global (#220)

Assigning a >24B by-value struct-return into a GLOBAL lvalue dropped the
struct body: the sret dest was routed to a BP scratch temp and only the
8-byte return pointer was stored (`MOVQ AX, g(SB)`); the callee wrote the full
struct to the scratch, which never reached the global. A BP-relative dest
offset cannot name a global symbol. Pre-existing GATE-BLIND silent miscompile
— both stages emit the same broken store, so byte-id (990-997) stays green
while runtime is wrong — latent until the eFinal io surface put a global
`cgoutstream: memio.stream` (>24B) on the path, where it made cgen.ww's
self-built w6c_ww buffer every function body into a corrupt global (pos stayed
0) and emit prologue-only output.

Fix, both stages, byte-identical: route the sret dest pointer to the global
symbol so the callee writes the full struct through RDI straight into the
global. cstage adds cg_sret_dest_sym, mirroring the existing str/slice global
arm (skip the @sretscr scratch, emit `LEAQ masym(sym), DI`). wwstage carries
the lhs IDENT node (sretdestnode) and emits `LEAQ name(SB), DI` via emitsymname
— identical to cstage's symbol mangling, verified cs.s==ww.s on the probe and
across 990-997. #211-family (by-value struct + global/pointer), but a distinct
site: the cstage assignment-store into a global, not the wwstage call-return.

N_LET-global static-init (`let g: T = mk()` at top level) is a separate,
independently-broken path (#221) — link-fails for init-via-call, returns 0 for
constant init — not the sret-receive gap and not on the eFinal path; deferred.

test/wcc/940_global_sret_run: global assign (plus a branched callee to defeat
const-fold), through-pointer mutation (the io vtable-callback shape that
surfaced this), and local-init/assign regressions — runtime asserts on both
stages (the net, since byte-id is gate-blind here) plus cs.s==ww.s.
Discrimination confirmed by revert+rebuild: with the global arm disabled,
global_assign emits the truncated store and exits 1.

selfhost/cmd/wcc/cgen.ww

@@ -505,6 +505,11 @@ type cgen = struct {
 	// cgenexpr resolve `@sretarg` via localfind when they need the
 	// saved RDI.
 	sretdestoff: i32,
+	// #220: sret receive into a GLOBAL lvalue. A BP-relative i32
+	// (sretdestoff) can't name a top-level let, so the lhs IDENT node
+	// is carried and emitted as `LEAQ name(SB), DI`. nil means no
+	// global receiver wired; mutually exclusive with sretdestoff.
+	sretdestnode: *node,
 	sretforward: i32,
 };
 
@@ -528,6 +533,7 @@ fn cgeninit(c: *cgen) void = {
 	c.labelseq = 0;
 	c.varargseq = 0;
 	c.sretdestoff = 0;
+	c.sretdestnode = nil;
 	c.sretforward = 0;
 	// Note: strlit_seq, strlits, ffis are *not* reset here; they
 	// persist across cgfn calls within one file. cgfile resets them