selfhost/cmd/wcc/check: flatten tagged spreads + iserror on tinfo.params (#61a)

A.6.3 #61 prerequisite (additive, no consumer changes). The tagged-variant
machinery (taggedvariantindex / flatvariant* / cgwidentagremap / cgmatch)
is AST-keyed -- it walks N_TTAGGED.list and spread-flattens `...inner` at
read time. To migrate it onto tinfo.params (#61b/c) the chain must first
carry the flattened variant set + per-variant error mark, matching cstage's
Type.params / Type.iserror.

tinfofornode's TTAGGED arm now splices `...inner` tagged spreads into
ti.params (dealias one NAMED level, require TY_TAGGED, inline its already-
flattened variants in declaration order) -- mirror of cstage check.c:366-389.
Each variant gets an iserror flag via varianterr (TBANG / `!`-aliased).
size/align stay accounted off the surface member so ti.size is byte-identical
to before; the flatten + iserror have zero readers this commit (the lone
TY_TAGGED params reader, nullableptrtag, only fires on 2-variant nullable
unions with no spreads).

iserror rides the shared tparam struct rather than a sidecar: a cstage-mirror
divergence from harec, which carries no per-variant flag (models `!T` as a
STORAGE_ERROR type node, ref/harec/include/types.h:144, src/types.c:151-159).
Faithful port filed as #62. Spread-only flatten (cstage check.c:373 also
flattens non-spread anonymous-nested unions) is a known symmetry gap, inert
in bootstrap, tracked for #61b.

make test 133/133 (quiescent tree, byte-id 990-997 green).

lib/ww/typ.ww

@@ -71,6 +71,15 @@ type tfield = struct {
 type tparam = struct {
 	name:   str,
 	type_:  *tinfo,
+	// #61a: per-variant `!T` error mark for TY_TAGGED variants.
+	// cstage-MIRROR divergence: harec carries no per-variant flag —
+	// it models `!T` as a distinct STORAGE_ERROR type node
+	// (ref/harec/include/types.h:144, src/types.c:151-159
+	// type_is_error). wwstage tinfo has no iserror field
+	// (check.ww TTAGGED arm), so the bit rides the shared param
+	// struct instead, matching cstage Type.iserror semantics.
+	// Faithful STORAGE_ERROR-node port filed as #62.
+	iserror: bool,
 	tnext:  *tparam,
 };