frame keeps: a model's records filled in place, kept event numbers, list spares, Json.read_file

From the fence's kept frames in 22 minutes of play:

- A component field that is a record or a list of records was `value_put(o, k, view_val_T(x))`,
  a whole new tree every frame (HudPrompt's notifications). view_fill.ludic generates view_set_T /
  view_set_list_T / view_fill_T that fill the object and list under the key in place.
- value_list_fit dropped the items it cut off and value_item made new ones as the list grew back,
  a Value per item per regrowth (value_item / value_set_strs); the cut-off items are now the
  list's spares (Val.spare), and an item of another kind is turned rather than replaced.
- ludic.ui: a scroll box's "scroll" and a slider's "change" fired a fresh Value.float a frame
  (sc_walk, scroll.ludic:36); ui_fire_float takes one from a ring kept with the state (fired.ludic).
  ui_object_fit_into is exported, for a draw that keeps its list.
- Json.read_file(path): read, parsed, and the file's text given back - Json.parse(Fs.read_text())
  kept the whole file on every read (Maroon Lake's settings peeks).

Golden value_list_regrow: a list alternating 6 and 2 items every frame keeps nothing (the toolchain
before this fails it: +128 B new Val from value_item). Game compiles; ludic.i18n/settings/hints/
base tests pass (ludic.ui has none); arena and fence goldens unchanged.

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

packages/ludic.ui/layout_fit.ludic

@@ -13,7 +13,8 @@ function ly_ratio(n: UiNode, fixed_w: float, avail: float) -> void {
 }
 # x, y, w, h: a `nat_w` x `nat_h` picture in the node's content box by its object-fit
 export function ui_object_fit(n: UiNode, nat_w: float, nat_h: float) -> []float { return ui_object_fit_into(new []float, n, nat_w, nat_h) }
-function ui_object_fit_into(out: []float, n: UiNode, nat_w: float, nat_h: float) -> []float {
+# ... into the caller's kept list (a draw every frame should: ui_object_fit makes one a call)
+export function ui_object_fit_into(out: []float, n: UiNode, nat_w: float, nat_h: float) -> []float {
   let bx = n.x + n.pl + n.bl
   let by = n.y + n.pt + n.bt
   let bw = n.cw - ly_hin(n)