runtime: Json.write_file, and Json.encode through a kept buffer

The encoder appends into RtJsonState's buffer (grown only past the biggest document yet): ints and
Q16.16 fixeds written as digits in place, floats through string() and freed. Json.encode copies the
answer out once; Json.write_file hands the buffer to Fs.write_text (.tmp + rename) and keeps nothing.
json_saves.ludic: exact text, the file equals encode, parse round-trips, 1000 saves grow 0; clean
under MallocScribble. json_quote (the + builder) is gone.

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

changes/json-write-file.md

@@ -0,0 +1,7 @@
+bump: minor
+type: feature
+**`Json.write_file(value, path)` saves without keeping anything.** It writes the text
+`Json.encode` would give straight to the file (through `path.tmp` and a rename), built in a buffer
+the runtime keeps, so a game that saves often holds nothing more for it. `Json.encode` itself is
+built the same way and makes only the string it returns: it used to join its text with `+`, keeping
+every piece and every number's text. `examples/lang/json_saves.ludic`: 1000 saves, 0 bytes.