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/scroll.ludic

@@ -32,7 +32,7 @@ function sc_walk(ui_st: mut UiState, n: UiNode, live: bool) -> void {
     var off = held
     if live { off = sc_input(ui_st, n, off, room, most) }
     off = Math.clamp(off, 0.0, most)
-    if live and off != Math.clamp(held, 0.0, most) { ui_fire(ui_st, n, "scroll", Value.float(off / ui_px(ui_st, 1.0))) }
+    if live and off != Math.clamp(held, 0.0, most) { ui_fire_float(ui_st, n, "scroll", off / ui_px(ui_st, 1.0)) }
     sc_put(ui_st, n.key, off)
     n.scroll_y = off
     if off != 0.0 {