v0.7.2 — zoom in and still know where you are

Fixed

• At full zoom the city now tells you what you are looking at. v0.7.1 stopped
  the camera dollying inside the buildings, and that held. But what replaced the
  blank frame was a wall of buffer-pool tiles with nothing naming them —
  legible only to someone who already knew that was shared_buffers at page
  scale. The literal blank was gone and the disorientation was not, which is
  plausibly what the original reporter experienced. Close range now identifies
  what fills the view and registers itself as a mode with a documented way back.
• One camera distance floor instead of two. MIN_DOLLY_DIST guarded the
  wheel at 24 while MIN_DIST let six other paths reach 8, and the measured
  blank range starts near 16 — so the shipped fix held only because nothing
  happened to point the camera that close. A test guarded it from the outside;
  now a single constant means the mistake cannot be written.