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).

internal/wwfixture/types.ww

@@ -1,13 +1,13 @@
 package wwfixture;
 
 def protocolversion: i32 = 1;
-def corpuscount: i32 = 1754;
+def corpuscount: i32 = 1758;
 def errorcount: i32 = 351;
 def compilecount: i32 = 21;
 def runcount: i32 = 209;
-def runexitcount: i32 = 1173;
-def nativecount: i32 = 3508;
-def corpushash: str = "67a119b41fd78b1f2900c096e5b5670e98935d2965573f67bb21ab817931dffa";
+def runexitcount: i32 = 1177;
+def nativecount: i32 = 3516;
+def corpushash: str = "ab5ae796c10e476af0c1f40e0d0b216b3eb708f62b6ee17aafcf79d5e1dd8a69";
 
 type directive = enum i32 {
 	ERROR = 0,