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>

examples/library/ui_first_show_parts/Cell.lss

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+.cell { width: 100px; height: 40px }

examples/library/ui_first_show_parts/Cell.ludic

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+component Cell {
+  prop caption: string = ""
+}

examples/library/ui_first_show_parts/Cell.xml

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+<div class="item">
+  <button class="cell" on-click="emit click"><p>{caption}</p></button>
+</div>

examples/library/ui_first_show_parts/Host.lss

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+.t { width: 60px; height: 30px }

examples/library/ui_first_show_parts/Host.ludic

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+component Host {
+  state on: bool = false
+  on flip() { on = not on }
+}

examples/library/ui_first_show_parts/Host.xml

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+<div>
+  <button class="t" on-click="flip()">t</button>
+  <if test="{on}"><Cell caption="a"/></if>
+</div>