Json.free_all is safe on a parsed tree a migration and a loader have edited; sv_str keeps a copy

A load freed only the parsed trip's nodes (Json.free) and kept every string the parser made, because
a loader might keep one; free_all freed every string and every key, so on a tree a migration had
added a literal to (`Value.put(v, "sver", Value.str("2"))`) it freed the literal and aborted.

- The parser marks the string values it makes (JP_OWNED, in the node's otherwise unused num) and
  interns object keys (a few names, never freed); free_all frees only marked strings, never keys,
  and a list's spares too. A setter that gives a marked node other text (value_set_str/_strs,
  value_into_str/_strs, value_become) frees the parser's text first. value_as_int / _as_float read a
  string as 0 as before.
- ludic.base sv_str returns intern(...): every package load that keeps a text read from a section
  (minimap labels, a Thing's look, photo tags and files, an effect's label, ...) holds its own copy.

Golden json_free_edited: parse, put a literal key and string in, set a string, keep an interned copy,
free_all - 1100 loads: the copy reads on and nothing grows (the toolchain before this aborts, 134).
Tests: ludic.save, base, settings, minimap, things, photo, effects; json_saves, value_list_regrow; the
36 ui examples.

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

examples/lang/json_free_edited.ludic

@@ -0,0 +1,24 @@
+# json_free_edited.ludic - a parsed file let go whole (Json.free_all) after it was migrated and read:
+# a literal key and a literal string put in, a string set in place, a copy kept - only what the
+# parser made is freed, so nothing crashes, the copy reads on, and a thousand loads keep nothing:
+#   names Sam|Ada grew 32
+program JsonFreeEdited {
+  function read_one(text: string, keep: []string) -> void {
+    let v = Json.parse(text)
+    Value.put(v, "sver", Value.str("2"))
+    value_set_str(v, "name", "Ada")
+    if len(keep) < 2 { push(keep, intern(Value.as_str(Value.get(v, "who")))) }
+    Json.free_all(v)
+  }
+  entry {
+    let keep = new []string
+    let text = "{\"name\": \"Sam\", \"who\": \"Sam\", \"pins\": [\"a\", \"b\"], \"n\": 3}"
+    read_one(text, keep)
+    push(keep, "Ada")
+    for f in 0 .. 1100 {
+      read_one(text, keep)
+      Mem.frame()
+    }
+    print(`names {keep[0]}|{keep[1]} grew {Mem.kept()}`)
+  }
+}