feat(cmd): GOOS-aware headers, audit battery shapes and semicolon spacing

Assisted-by: GLM 5.3 Flash

format/format_test.go

@@ -364,6 +364,17 @@ func TestSemicolonSeparators(t *testing.T) {
 			in:   "TEXT ·f(SB), $0\nBYTE $1;\nRET\n",
 			want: "TEXT ·f(SB), $0\n\tBYTE $1;\n\tRET\n",
 		},
+		{
+			// The REP shape: a prefix-style zero-operand statement
+			// followed by the instruction it prefixes.  The separator
+			// belongs to the statement it ends, so the function's
+			// alignment width (MOVSQ is the widest mnemonic here) must
+			// not open a gap before it: one space after the mnemonic
+			// whatever the neighbours' lengths.
+			name: "after a prefix-style statement",
+			in:   "TEXT ·f(SB), $0\nMOVQ AX, BX\nREP; MOVSQ\nRET\n",
+			want: "TEXT ·f(SB), $0\n\tMOVQ AX, BX\n\tREP ; MOVSQ\n\tRET\n",
+		},
 	}
 	for _, tc := range cases {
 		t.Run(tc.name, func(t *testing.T) {