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>

docs/language/json/json-write_file.md

@@ -0,0 +1,26 @@
+---
+id: json-write_file
+name: Json.write_file
+category: json
+kind: namespace-method
+tokens: Json.write_file
+sig: Json.write_file(value, path) -> bool
+tip: Write a value tree to a file as JSON, keeping nothing.
+order: 5
+ns: Json
+member: write_file
+---
+
+Writes the tree as the same text <a href="json-encode"><code>Json.encode</code></a> would give, straight to <code>path</code>: to <code>path.tmp</code> first and then renamed over it, as <code>Fs.write_text</code> does, so a crash mid-write leaves the previous file whole. The text is built in a buffer the runtime keeps and grows only for a document bigger than any before, so a game can save as often as it likes and hold nothing more for it - where <code>Json.encode</code> hands back a string of its own every time. False when the file cannot be written.
+
+```ludic
+program Save {
+  entry {
+    let o = Value.object()
+    Value.put(o, "day", Value.int(3))
+    let path = Os.temp_dir() + "/save_doc.json"
+    print(Json.write_file(o, path))
+    Fs.remove(path)
+  }
+}
+```