wcc: case T => variant validity check (C + selfhost)

`match (u) { case T => ... }` where T isn't a variant of u was
silently accepted by both checkers. The cgen would emit a tag
comparison against an index that never appears, leaving the arm
unreachable — wasted code that's almost always a bug or typo.

C check.c now mirrors the existing is/as rule for match arms:
each `case T` and each alt of multi-pattern `case T1 | T2` is
checked against the scrutinee's variant list via variant_present.

selfhost check.ww gets the same shape with AST-level type_eq_ast
comparison. Both checks land in the same scope-aware pass that
already runs exhaustiveness and ? subset.

New test rows in 300_check (C side) and 950_selfcheck (selfhost
side) exercise both single-pattern and multi-pattern alt typos.
The 950 driver's err_present detector picks up the new
"case: not a variant" prefix.

cmd/wcc/check.c

@@ -909,6 +909,28 @@ cexpr(Checker *c, Node *n)
 				cs->type = vt;
 				for (Node *alt = cs->list; alt; alt = alt->next)
 					alt->type = resolve_type(c, alt);
+				/* Validity: every `case T =>` pattern must
+				 * refer to a variant of the scrutinee's
+				 * tagged union. Mirrors the existing is/as
+				 * check; `match (u) { case f64 => ... }`
+				 * where f64 isn't a variant of u is dead code
+				 * the dispatch never reaches, so refuse it. */
+				if (vt && vt != ty_err &&
+				    !variant_present(u->params, vt))
+					err(c, cs->pos,
+					    "case: %s is not a variant of %s",
+					    type_name(c->a, vt),
+					    type_name(c->a, st));
+				for (Node *alt = cs->list; alt; alt = alt->next) {
+					if (alt->type == NULL ||
+					    alt->type == ty_err) continue;
+					if (!variant_present(u->params,
+					    alt->type))
+						err(c, cs->pos,
+						    "case: %s is not a variant of %s",
+						    type_name(c->a, alt->type),
+						    type_name(c->a, st));
+				}
 				if (cs->str && cs->str[0])
 					scope_define(c->cur, cs->str, SK_VAR, vt, cs);
 			}