feat(ui): popovers, tooltips, progress; the render3d backend installs itself; duplicate names caught first

- `popover`: a top layer that keeps the pointer and the keyboard, closed by a press outside or Esc.
- `title` tooltips after half a second's rest, styled by .ui-tooltip.
- `<progress>` and `<meter>`.
- Atlases take rows and number cells, and importing ludic.ui/render3d.ludic is enough to draw with
  render3d.
- The compiler reports two declarations of one name before it type-checks, so a package global
  clashing with a program's reads as that, not as 29 type errors.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

selfhost/main.ludic

@@ -203,6 +203,8 @@ entry {
   else { g_windowed = has_target and has_systems() }
 
   g_parsing = false                               # lowering errors now locate by statement, not token
+  check_duplicate_fns()                           # two declarations of one name: the cause, before its symptoms
+  check_duplicate_decls()
   check_program()                                 # L4: the types agree before anything is emitted
   emit_program()