apparently the laptop also has an eSATA/USB2.0 port (originally thought it was just eSATA, but learned that it's both). Plugging in the USB CD drive to that port allows the installer to boot successfully.
retitled bug to emphasize USB 3.0; probably not HP Envy 15 specific either, but don't have another machine with USB3.0 ports to test with.
wow, more fun;
apparently the laptop also has an eSATA/USB2.0 port (originally thought it was just eSATA, but learned that it's both). Plugging in the USB CD drive to that port allows the installer to boot successfully.
retitled bug to emphasize USB 3.0; probably not HP Envy 15 specific either, but don't have another machine with USB3.0 ports to test with.