fix(ui): collect after a rebuild so the heap is in a known state

A build allocates a spec table per node and drops them all at once, so an
app that scans WiFi right after a screen change met whatever the incremental
GC had got around to. Costs a few ms on a screen change; recovers ~24 KB.

lua/lib/ui.lua

@@ -386,6 +386,11 @@ function ui.rebuild()
   laidOut = true
   gui.clear(ui.theme.background)
   node.draw(root)
+  -- A build allocates a spec table per node and drops them all here, and the next thing an
+  -- app does may be the one that needs a contiguous WiFi buffer. Collecting now costs a few
+  -- milliseconds on a screen change nobody can see, and leaves the heap in a known state
+  -- instead of one that depends on when the incremental GC last ran.
+  collectgarbage()
 end
 
 function ui.draw()