wcc: checker-seam drain — spreads, tuple aliases, yield scoping

Four seams from one review cluster, all landing on the same checker
files; gates ran on the union (rule 11 body).

?-subset: the wwstage walk compared `...spread` ALIAS nodes on both
sides of the error-subset check, falsely rejecting a spread-carried
error return (`(i64 | ...errs)`, cs-accept/ww-reject) — both sides
now flatten like trycountvariants (cstage Tparams are pre-flattened
at type level).

Multi-let/multi-assign: the tuple gates keyed on the RAW rhs kind,
rejecting a NAMED tuple alias (`type pair = (i64,i64)`; cs clean-
reject, ww asserttyped stop) — both stages chase per #99 alias
transparency, wwstage rettupleof peels the alias so cgmassign keeps
the str element's len/cap stores, and the catA massign-alias error
pin re-rules to a run fixture. The N_MLET diagnostic also printed
its operands swapped (elem/declared reversed vs its wording).

Yield: match_yield_type didn't descend into N_SWITCH, so a yield
inside a switch arm typed the match void and dropped the value
(both stages); and a stray yield outside any match arm reached cgen
unchecked — both stages now gate on a match-arm counter (the
c->loops discipline).

cmd/wcc/ww.h

@@ -584,6 +584,7 @@ struct Checker {
 			 * directives when refusing param/let names that
 			 * would shadow an imported module bareword. */
 	int	loops;	/* nesting count for break/continue */
+	int	matcharms;	/* nesting count for yield */
 	int	errs;
 	int	is_test;	/* #15: `w6c -T` — collect @test fns + synth
 				 * the entry; loud-reject a user main. */