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/backend.ludic

@@ -51,6 +51,8 @@ export state UiState {
   ev_ring_at: int = 0
   nt_evs: []Val = nt_evs_new()          # native.ludic: fired events' objects, NT_EVS of them, made with the state
   nt_ev_at: int = 0
+  nt_nums: []Val = nt_nums_new()        # fired.ludic: the numbers a scroll or a slider reports, a ring
+  nt_num_at: int = 0
   hd_ev: Val = hd_ev_new()               # hold.ludic: the held control's event, filled in place
   cv_texts: []string = new []string      # units_pieces.ludic: a value's var() pieces, kept
   tx_pieces: [][]string = new [][]string # eval_text.ludic: a text's pieces, a list a depth