feat: Growable heap, GUI-by-default run, live sysmon, SUB/SUB default login

Kernel / memory:
- mm/kmalloc.c: fix "Heap: Out of memory" under the GUI. The desktop allocates
  three full-screen 32-bit buffers (~11MB) which overflowed the fixed 16MB pool.
  The heap now starts at 32MB and grows on demand from the PMM (16MB regions),
  with region-safe coalescing (only merges physically adjacent blocks) so growth
  never corrupts across separate PMM allocations. Adds heap_get_grow_count().

Build / run:
- Makefile: merge `run-gui` into `run` so `make run` boots into the graphical
  desktop; add `run-headless` (serial-only via QEMU_CMD), bump guest RAM to
  256M, update .PHONY and help.

Configure:
- scripts/kconfig/menuconfig.py: gate the curses UI behind isatty() on stdin and
  stdout so non-interactive `make configure` stops emitting "nocbreak() returned
  ERR" and falls cleanly through to the non-interactive writer.

GUI / userland:
- gui/gui_apps.c: rebuild the System Monitor from hardcoded values into live
  telemetry -- real heap used/total/free with a colour-shifting bar, a scrolling
  heap-utilisation graph, the grow counter, and real PMM RAM figures.
- lazybox `free`: report how many times the heap has grown.

Auth:
- Default login is now SUB / SUB (Kali-style username==password), seeded first
  as the primary uid-0 account and set as the logged-in user; root kept for
  compatibility. Hardcoded "root" defaults (whoami, su, passwd, ssh, sshd
  session) updated to SUB.

Verified: clean build on x86_64/aarch64/armv8i; GUI boots with 0 OOM/0 panic;
auth engine initialises cleanly; `make configure` fallback is clean.

Co-Authored-By: Claude Opus 4.8 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Fh5z6V7nGPbHvqVG5p4CXX

scripts/kconfig/menuconfig.py

@@ -829,17 +829,26 @@ def main():
             print(f"menuconfig: lxdialog front-end failed ({exc}); "
                   f"falling back to the curses UI.", file=sys.stderr)
 
-    # Fallback to Curses GUI replica
-    try:
-        import curses
-        curses.wrapper(run_curses_gui)
-        print(f"*** Configuration saved to {CONFIG_FILE} and {AUTOCONF_HEADER} ***")
-        return
-    except Exception as exc:
-        print(f"menuconfig: curses UI unavailable ({exc}); "
-              f"writing the current configuration non-interactively.", file=sys.stderr)
+    # Fallback to the curses GUI replica, but only when we genuinely have an
+    # interactive terminal on both ends. Probing curses without a real TTY makes
+    # it fail deep inside the wrapper's teardown (e.g. "nocbreak() returned
+    # ERR"), which looked like a crash; gate on isatty so a piped or CI
+    # invocation drops straight to the non-interactive writer instead.
+    if sys.stdin.isatty() and sys.stdout.isatty():
+        try:
+            import curses
+            curses.wrapper(run_curses_gui)
+            print(f"*** Configuration saved to {CONFIG_FILE} and {AUTOCONF_HEADER} ***")
+            return
+        except Exception as exc:
+            print(f"menuconfig: curses UI unavailable ({exc}); "
+                  f"writing the current configuration non-interactively.", file=sys.stderr)
+    else:
+        print("menuconfig: no interactive terminal detected; "
+              "writing the current configuration non-interactively.", file=sys.stderr)
 
-    # Last resort: no usable TTY at all.
+    # Last resort: no usable TTY at all. Preserve an existing .config if present,
+    # otherwise fall back to the architecture default so the build never stalls.
     cfg = load_config()
     save_config(cfg)
     print(f"*** Configuration written to {CONFIG_FILE} and {AUTOCONF_HEADER} ***")