fix(parser): reject macro parameter lists the toolchain rejects

The C variadic spellings ("..." and the GNU "name..."), an empty or
trailing parameter, a missing comma, a stray token and an unterminated
list all parsed silently before: non-identifier tokens were skipped, so
"#define M(...)" defined a zero-parameter macro and invocations were
diagnosed only by argument count, if at all.  The toolchain rejects the
definition itself ("bad definition for macro"), and so does the
preprocessor now: the parameter list must be identifiers separated by
single commas and closed by the parenthesis, and a rejected definition
leaves the name unbound.  The spellings join the fuzz corpus.

Assisted-by: GLM 5.3

parser/fuzz_test.go

@@ -94,6 +94,8 @@ func FuzzParseExpand(f *testing.F) {
 	f.Add("#include \"no/such/header.h\"\n")
 	f.Add("#line 7 \"f.s\"\n")
 	f.Add("#define M(x) \\\n\tADD $x, R0 \\\n\tSUB $x, R1\nM(1)\n") // continuations
+	f.Add("#define M(...) x\nM(1)\n")                               // the C variadic spelling, a bad definition
+	f.Add("#define M(args...) x\nM(1)\n")                           // the GNU variadic spelling, one dotted name
 	// A body that repeats its argument multiplies every nesting level; the
 	// expansion must stop at the work budget, not produce the exponential
 	// result it names.