w6c,ww: re-emit ... union-spread marker in .wwi producer (M4 E3, #95)

The N_TTAGGED serializer emitted each variant via wwi_type but never
re-emitted the `...` prefix for TK_ELLIPSIS spread variants, so an
exported `(...inner | str)` round-tripped through .wwi as `(inner | str)`.
The consumer's checker then could not flatten inner's members into the
alias and variadic-assignability rejected bare members — under separate
compilation this broke fmt/log/getopt. Re-emit `...` before the variant
type, both stages; the producer stays purely syntactic (flatten/dedup
remain the consumer's type-store job, per ref/hare/hare/unparse/type.ha:290-300).

Gate: test/wcc/989_wwispread_sep.c — table-driven (2-arm + 3-arm spreads)
x both stages, asserts the marker survives the .wwi, the consumer binds
bare members under --sep (exit 0), and cs==ww .wwi byte-identity.

test/wcc/data/wwispread/main.ww

@@ -0,0 +1,13 @@
+package main;
+
+import liba;
+
+// Each assignment binds a BARE member of inner (i32, i64) or the outer-direct
+// arm (str) to liba.outer — typechecks ONLY if the spread flattened inner's
+// arms into outer across the liba.wwi round-trip (#95).
+export fn main() void = {
+	let x: liba.outer = 42i32;
+	x = 99i64;
+	x = "hi";
+	return;
+};