v3.1.1

A nav nested three levels deep put a <ul> directly inside a <ul>, with no <li> between
them. Invalid HTML, and the counts a screen reader reads out of a list come from that
nesting — so the third level and everything under it was announced wrong. Two levels were
fine, which is why nothing caught it: the mock site stopped at two, so script/a11y never
rendered the markup and reported green over a branch it had never seen.

Fixed

• navtree.html emits a valid list at any depth. The per-node <li> moved into its own
  navnode macro and the recursion goes through that, so a nested section arrives as an
  <li> rather than as the <ul> the old navtree call opened with. Only sites with three
  or more nav levels produced the broken markup; their DOM changes, everything shallower is
  byte-identical.

Added

• A third nav level in the mock site. preview/src/docs/guide/deep/ — a section inside
  Guide and one leaf under it. Filler pages, but they are what makes the recursive branch
  of navtree.html render at all, and reverting the fix now fails the sweep with
  list (serious) on every page instead of passing.

Full Changelog: v3.1.0...v3.1.1