cgen: resolve chained-DOT fn-ptr field callees

fnptrcalleetfn's N_DOT arm required an N_IDENT base, so a.b.cb(...)
fell to the name registry with an empty module hint and emitted
CALL cb(SB) (undefined symbol; cstage calls the stamped ptr indirect).
dotbasestructinfo resolves the base chain through the struct registry
— each link a struct- or *struct-typed field — and the single-dot
path routes through the same resolver unchanged. Closes the last open
shape of the #59.8 name-keyed callee family. Fixture fnptrfield_chain;
corpus pin 1487/2974.

internal/wwfixture/types.ww

@@ -1,13 +1,13 @@
 package wwfixture;
 
 def protocolversion: i32 = 1;
-def corpuscount: i32 = 1486;
+def corpuscount: i32 = 1487;
 def errorcount: i32 = 338;
 def compilecount: i32 = 17;
 def runcount: i32 = 191;
-def runexitcount: i32 = 940;
-def nativecount: i32 = 2972;
-def corpushash: str = "ab4b1d1ffc078b23de0a70911740511e873cd8d6f76009c4719d704bb36e6133";
+def runexitcount: i32 = 941;
+def nativecount: i32 = 2974;
+def corpushash: str = "2520ada0b1687fab6eed6073a263c620fa4cbab6dc8156f7d51bcf8b14a9ec4a";
 
 type directive = enum i32 {
 	ERROR = 0,