fix(format): keep macro content behind a label on its line

The canonical form splits a label from the instruction that follows it,
but an identifier naming a macro may expand into any token at all: moved
to a line of its own it no longer parses, because the parser accepts a
non-mnemonic first token only behind a label.  The names #define'd in
the file now hold such content back, conservatively across the whole
file, and the crashing input joins the corpus.

Assisted-by: GLM 5.3

format/expansion_test.go

@@ -107,8 +107,22 @@ func TestFormatPreservesExpansionSemantics(t *testing.T) {
 				"\tRET\n",
 		},
 		{
-			// A use before the define: the name is not a macro yet at that
-			// point of the file, so the label splits as any other does.
+			// Content after a label that names a macro: the expansion may
+			// start with any token, so the canonical form keeps it on the
+			// label line, where the parser accepts a non-mnemonic first
+			// token behind a label but not at the start of a line.
+			name: "macro behind a label",
+			src: "#define M 0\n" +
+				"TEXT ·f(SB), $0\n" +
+				"\tA: M\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
+			// the formatter keeps is deliberately conservative and leaves
+			// the line whole, which preserves the expansion in both
+			// directions alike.
 			name: "label before the define of its name",
 			src: "TEXT ·f(SB), $0\n" +
 				"\tM: A00\n" +