play 2's frame keeps and a dropped file: a var() join made at once, the hint rail emptied in place, save_read gives its text back

- ludic.ui cv_join (units_pieces.ludic:93): a var() value's pieces were joined by +, a text left
  behind per piece on every memo miss; it is written at its length at once (units_join.ludic), and
  the miss is its own declared function (cs_vars_miss), bounded by the memo.
- ludic.hints hn_slots (rail.ludic:41): hints_reset made a new rail every time; the rail is
  emptied in place, made again only for another count (rail_slots.ludic).
- value_spare_put (value.ludic:138): a list's spares grow only past the most it has ever cut off;
  declared as such.
- ludic.save: save_read reads, parses and frees the file's text - the tree's strings are the
  parser's own - and says whether the text was whole (SaveRead.intact) for the words of a refusal.
  Maroon Lake's trip and home reads kept the whole file on every load and every menu card read.

Tests: ludic.save (10), ludic.hints (8); the 36 ui examples; value_list_regrow, json_saves and
alloc_fence_declared unchanged.

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

packages/ludic.ui/units.ludic

@@ -68,6 +68,11 @@ function cs_vars_in(ui_st: mut UiState, n: UiNode, v: Val) -> Val {
     if m.tags[i] == MM_VARS and cv_same(ui_st, m.keys[i]) { return m.vals[i] }
     at = (at + 1) & (MM_CAP - 1)
   }
+  return cs_vars_miss(ui_st, m)
+}
+# a value's var()s resolved to something not seen before: built once and kept in the memo
+@alloc_ok("a memo miss: a var() value first seen, then kept, up to MM_CAP")
+function cs_vars_miss(ui_st: mut UiState, m: UiMemo) -> Val {
   let raw = cv_join(ui_st)
   let r = Value.str(tpl_words(raw))
   let j = mm_add(m, MM_VARS, raw)