Os.heap_bytes: the heap without the renderer; string_temps reads it

Vk.heap_bytes pulled the Vk module - and on lang/uifree the GL window path - into a plain program,
which then failed to link (_cgl_offscreen, lgl_GetError). Os.heap_bytes is malloc_zone_statistics
through a weak reference (0 where there is none, and on Windows). Reseeded. Docs for it and for
Json.free / Json.free_all.

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

changes/string-temporaries.md

@@ -9,3 +9,5 @@ variable, an argument, a function's result - is left alone. A loop of 20,000 tem
 2.9 MB and now grows 0 (`examples/lang/string_temps.ludic`).
 A slice of a string (`s[a .. b]`, always a copy) counts too: `path[0 .. n - 4] + ".dds"` and
 `path[n - 4 .. n] == ".png"` no longer leave the slice behind.
+`Os.heap_bytes()` reads malloc's live bytes (macOS; 0 elsewhere) with no renderer linked in, for a
+test that a path allocates nothing.