fix(format): treat a selector-folded label as naming its macro

The lexer folds NAME.selector into one identifier, and an object macro
reached through the selector expands with it travelling along, so a
label spelled NAME.selector: restructures exactly like the bare name
would.  The macro-name checks now resolve the prefix before the first
period, and the crashing input joins the corpus.

Assisted-by: GLM 5.3

format/expansion_test.go

@@ -127,6 +127,17 @@ func TestFormatPreservesExpansionSemantics(t *testing.T) {
 				"\tA: M:\n" +
 				"\tRET\n",
 		},
+		{
+			// A label reached through a selector: the lexer folds NAME and
+			// .selector into one identifier, and the object macro behind the
+			// name expands with the selector travelling along, so the folded
+			// spelling restructures exactly like the bare name.
+			name: "label naming a macro through a selector",
+			src: "#define A B\n" +
+				"TEXT ·f(SB), $0\n" +
+				"\tA.: A0\n" +
+				"\tRET\n",
+		},
 		{
 			// A use above its define: the name is not a macro at that point
 			// of the file, so splitting would be safe; the whole-file set