ludic.ui: a new component instance's first build is declared - a first show mid-play is not a frame's keep

Master's windowed fog profile failed the fence at walk t 35 s: +512 B from value_new / value_put /
value_slot / value_lists under bd_class and cmp_keycap_model - a KeyCap first shown mid-walk.

It is a first build, bounded, not a per-show leak: an unmounted instance goes to in_free and the next
show of its class reuses it (in_reuse, `renew`), its props and model objects kept and filled in place.
Only a NEW instance makes them - bounded by how many of that class are up at once, and the ones
unmounted less than two builds ago. in_reuse already declared the record; the props and model objects
and their first fill, made afterwards in bd_class, were not. They are now: in_reuse marks a new record
`fresh`, and bd_class's first fill of it goes through bd_first_fill (@alloc_ok) - a reused instance's
fill stays judged, so a real per-show leak would still fail.

Golden ui_first_show: a component first shown at frame 700, once the fence is judging, then hidden and
shown twice more: bad 0 (the toolchain before this fails frame 701: +464 B value_new from value_slot
under cmp_cell_model, value_put grow - the profile's failure). The 37 ui examples pass.

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

packages/ludic.ui/instance.ludic

@@ -7,6 +7,7 @@ property UiInst {
   seen: int = 0
   props: Val = null             # its props, filled again each frame into the same object
   model: Val = null             # and its model, filled in place by the class's model()
+  fresh: bool = false           # made, not reused: its first build makes its props and model
 }
 # builds before an unmounted instance is mounted again: the tree that last named it is good for two
 const IN_SPARE_AFTER: int = 2
@@ -41,6 +42,7 @@ function in_reuse(ui_st: mut UiState, k: UiClass) -> UiInst {
   let it = new UiInst
   it.cls = k
   it.ptr = k.make()
+  it.fresh = true
   return it
 }
 # after a build of `screen`: its instances the build did not reach are unmounted