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/index.ludic

@@ -16,6 +16,7 @@ import "memo_more.ludic"
 import "memo_join.ludic"
 import "eval_text.ludic"
 import "units_pieces.ludic"
+import "units_join.ludic"
 import "expr.ludic"
 import "expr_parse.ludic"
 import "expr_prim.ludic"