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/testdata/fuzz/FuzzFormatExpansionRoundTrip/ea7fb86b86361801

@@ -0,0 +1,2 @@
+go test fuzz v1
+string("#define A B\"\nA.:A0")