wwstage: name vararg-gather slots via mklabel; graduate fmt 777/780/781 (#227)

wwstage named variadic-gather slots with mkvarargname off a separate
varargseq counter, never bumping the shared labelseq that names match
labels. cstage names them via mklabel (cmd/w6c/cgen.c:5427,5431), which
advances labelseq twice per gather. So by the time main.main reached its
`match (wr)`, wwstage's match-label counter ran two behind cstage's
(_4/_5/_6 vs _6/_7/_8) — a pure label-numbering divergence that kept fmt
cs/ww byte-id failing.

Drop varargseq and the mkvarargname helper; call the existing mklabel
for the two gather slots, matching cstage's order (vararg_d only when
nvar>0, vararg_sl always). The slot names are locals-table keys only —
they resolve to BP offsets and never reach the asm — so only the
labelseq advance is observable, which is exactly what realigns the
downstream match labels. cstage untouched (align wwstage up).

This was the match-label half of fmt's divergence; with the earlier
compound-assign fix it completes fmt byte-identity. Graduate
777/780/781 to STAGE_CS|STAGE_WW with byte_id, and drop the now-stale
(void)asm_byte_identical guard in 777.

selfhost/cmd/wcc/cgen.ww

@@ -491,13 +491,6 @@ type cgen = struct {
 	yieldbuf:    []str,	// stack of match end labels for yield
 	defertop:    i32,
 	deferbuf:    []*node,	// stack of deferred exprs (LIFO at return)
-	// Variadic-call gather state. cgcall bumps this on each gather
-	// emit and uses it to mint `@vararg_d_N` / `@vararg_sl_N` per
-	// callsite; mirrors cstage's mklabel("vararg_d/sl") freshness
-	// so two variadic callsites with different arities in one fn
-	// get distinct slots (the shared slot fail-louds under #15's
-	// @-prefix grow-on-pin discipline).
-	varargseq: i32,
 	// System V AMD64 sret discipline (#23). Plain TY_STRUCT returns
 	// with size > 24B are passed via a hidden first-arg pointer
 	// (RDI) to a caller-prealloc dest; the callee writes through
@@ -547,7 +540,6 @@ fn cgeninit(c: *cgen) void = {
 	c.frame = 0;
 	c.lastwasreturn = 0;
 	c.labelseq = 0;
-	c.varargseq = 0;
 	c.sretdestoff = 0;
 	c.sretdestnode = nil;
 	c.sretforward = 0;