selfhost+test: resolve aliased tagged in taggedvariantindex (#20)

wwstage's taggedvariantindex returned -1 (caller maps to 0) for
N_IDENT returns of an aliased mixed-variant union. Cstage returned
the correct variant index. Cross-stage divergence — root cause of
worker-fmtparser's "reads bool-true as false" symptom in the #18
repro chain. Worker-18 dodged it by dropping 707's asm byte-id
loop; #20 re-enables it.

Unwrap at entry: resolvetagged peels N_TNAME alias chains down to
the underlying N_TTAGGED before the variant-index walk. Direct-
tagged callers are unchanged (resolvetype is a no-op on non-N_TNAME).
Mirrors nodeisstr's shape — same class of wwstage-no-typed-AST gap
tracked by #11.

Test 707 grows from 6 → 9 rows; new rows pin tag=0/1/2 (i64/str/
bool) explicitly so a future variant-reorder can't hide behind a
coincidentally-correct tag=0. Asm byte-identity loop re-enabled
(disabled by #18); now exercises both #18 (ABI words) and #20
(variant-index) fixes — rows 2/3/6 also probe str/bool divergence.

995_self_rebuild green confirms wwstage source itself has no
latent aliased-tagged-return that would have surfaced as a self-
divergence.

selfhost/cmd/wcc/cgenutil.ww

@@ -2200,6 +2200,18 @@ fn rhstargetname(c: *cgen, rhs: *node) str = {
 fn taggedvariantindex(c: *cgen, tagged: *node, rhs: *node) i32 = {
 	if (tagged == nil) { return -1; };
 	if (rhs == nil)    { return -1; };
+	// Alias-unwrap: wwstage has no typed AST, so an aliased tagged
+	// return (`type ft = (i64|str|bool); fn f() ft = ...`) reaches
+	// here as N_TNAME("ft"), not N_TTAGGED. flatvariantidx and the
+	// fallback both gate on N_TTAGGED → -1 → caller maps to 0,
+	// silently emitting `MOVQ $0, AX` for every non-leading variant.
+	// Cstage's check.c canonicalizes N_TNAME → underlying upfront;
+	// every wwstage cgen consumer of a type-bearing node has to
+	// remember this step itself. TODO(#11): a wwstage check pass
+	// between parse and cgen would replace the per-site unwrap with
+	// a single canonicalization. Same shape of fix as nodeisstr.
+	let resolved: *node = resolvetagged(c, tagged);
+	if (resolved != nil) { tagged = resolved; };
 	let wantname: str = rhstargetname(c, rhs);
 	if (wantname.len > 0) {
 		let r: i32 = flatvariantidx(c, tagged, wantname);