fix: stacks at top of user VA in own 1 MiB band (guard, heap kept below), 8/16 pages default; kernel fault msg names pid, kind, flags stack overflow; flaky OVERFLOW + spawntest/test-qemu check it

docs/ipc.md

@@ -79,9 +79,12 @@ the slots of the console, log and storage capabilities it was handed, and
 whether its parent put an arguments page at `ARGS_VA` (NUL-separated, the
 program's name first; the parent fills a page and `MapPage`s it over, so
 the program owns it and it goes when the program does). It sits at
-`0x1000`, above the unmapped page 0; the stack runs from just below the
-image (`0x10000`) down toward it: 4 pages for system programs unless init's
-table says more, 8 for apps.
+`0x1000`, above the unmapped page 0, below the image (`0x10000`). The
+stack is at the top of the user window (`types::layout`): it ends at
+`STACK_TOP` (1 GiB) in a 1 MiB band of its own, 8 pages for system
+programs unless init's table says more, 16 for apps; the rest of the band
+is never mapped (the heap stays below it), so running off the stack
+faults there and the kernel prints `stack overflow` with the process id.
 
 Boot loads only init and storage, both packed into the kernel image (they
 are what reads the disk, so they cannot come from it). The disk is one ext4