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>

docs/language/os/os-heap_bytes.md

@@ -0,0 +1,25 @@
+---
+id: os-heap_bytes
+name: Os.heap_bytes
+category: os
+kind: namespace-method
+tokens: Os.heap_bytes
+sig: Os.heap_bytes() -> long
+tip: malloc's live bytes (macOS; 0 elsewhere).
+order: 19
+ns: Os
+member: heap_bytes
+---
+
+The bytes malloc holds live across every zone of the process, on macOS; 0 on other platforms. Ludic has no garbage collector, so a path that runs every frame must give back all it takes: read this before and after many rounds of it, and the difference is what it keeps. A test built on it should allow for 0 elsewhere.
+
+```ludic
+program Heap {
+  entry {
+    let before = Os.heap_bytes()
+    var n = 0
+    for i in 0 .. 1000 { if `{i}` == "7" { n += 1 } }
+    print(Os.heap_bytes() - before)
+  }
+}
+```