w6c: key tagged natural-push call args on type equality, not slot size

pushargsrev's five aistagged gates (N_IDENT #55, N_CALL #21, N_INDEX
#12, N_DOT #22a, deref #35) treated a tagged arg as already-tagged
when its SLOT SIZE matched the param's. A same-slot subset union
((bool|void) into (i64|bool|void), both 16B) then natural-pushed the
narrower box's words carrying SOURCE tags — no re-layout, no
cg_widen_tag_remap twin — so the callee matched the wrong arm
(silent: probes exited 10/90 where cstage exits 30/27). cstage keys
widen detection on type equality (cgen.c:10000 same = (pu == au) ||
type_eq) and routes every non-same tagged source through the zeroed
scratch + tag remap; the slot-DIFFER wwstage path already mirrored
that byte-identically, so the fix computes cstage's same check once
(wsame) and replaces each slot-size test with it. This also erases
the last known cs!=ww shape divergence (the 16B-local staging vs
direct-push frame delta on prefix subsets).

8 subsetwiden_* fixtures own the class: call-result/ident/str-payload
/mid-arg remap (the wrong-arm shapes), prefix (the shape-divergence
repro), and bigslot/return-pos/struct-24-to-32 sibling guards. Corpus
pin 1740/343/22/209/1166/3480.

internal/wwfixture/types.ww

@@ -1,13 +1,13 @@
 package wwfixture;
 
 def protocolversion: i32 = 1;
-def corpuscount: i32 = 1732;
+def corpuscount: i32 = 1740;
 def errorcount: i32 = 343;
 def compilecount: i32 = 22;
 def runcount: i32 = 209;
-def runexitcount: i32 = 1158;
-def nativecount: i32 = 3464;
-def corpushash: str = "df3862c647aae744b549089ab4a180b36371fe898f5996c13099f52da203265e";
+def runexitcount: i32 = 1166;
+def nativecount: i32 = 3480;
+def corpushash: str = "a2f2514f07ca4e20457331e30f051394da3e59fcfa8f5f6ec3b33d9c14490354";
 
 type directive = enum i32 {
 	ERROR = 0,