selfhost+test: bump wwstage varargseq per cgcall (#8)

Latent surface from #15: cgcall variadic-gather block read seq from
n.uval, which post-#15 is always 0 because scanlocals (which used to
stamp it during pre-pass) was deleted. Every variadic callsite in a
fn aliased to @vararg_d_0 / @vararg_sl_0. When two callsites in one
fn had differing arities, the second hit #15's first-use+fail-loud
guard ("localadd: @-prefix slot grew within fn") — correctly, since
the slot was being asked to grow mid-fn.

Fix: read seq from c.varargseq + bump in cgcall's gather branch.
Mirrors cstage's mklabel("vararg_d/sl") natural seq bump.
cgeninit zeroes c.varargseq per-fn (existing), so the counter is
correctly per-fn scoped.

cgen.ww varargseq comment refreshed — replaces stale "bumped only at
emit time" misclaim with the post-#15 per-call shape + the #15
grow-on-pin discipline that surfaced the wedge.

751_vararg_seq_percall: table-driven 3 rows x 2 stages = 6 fixtures.
mixed_arity_two_calls (the wedge), same_arity_two_calls (no-regress),
three_arity_drift (1/2/3 mints @vararg_d_0/1/2).

make test 125/125; ww2==ww3==ww4 byte-id holds via 995_self_rebuild.

Surfaced by worker-strcontains2 attempting strings.contains tagged-
variadic graduation — mixed-arity spec test rows triggered the wedge.
Unblocks #9 + #10 (strings/bytes.contains).

selfhost/cmd/wcc/cgen.ww

@@ -469,10 +469,12 @@ 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 assigns per-call scratch
-	// names `@vararg_d_N` / `@vararg_sl_N` using this counter;
-	// post #15 the seq is bumped only at emit time so a single
-	// sequence is observed (the scanlocals pre-pass was dropped).
+	// 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