feat: process exit codes, posix_spawn, 4 KiB command lines

Groundwork for the GCC driver: a compiler driver must spawn cc1/as/ld
and know whether each stage succeeded.

Kernel: the scheduler keeps an exit-code ledger (pid -> code; pids are
monotonic so entries never alias), published during teardown right
before waiters wake. SYS_EXIT records main()'s return value, SYS_KILL
records 256+SIGKILL, and the exception handler records 256+signal
mapped from the fault vector (#PF/#GP -> SIGSEGV, #DE/FP -> SIGFPE,
#UD -> SIGILL). SYS_WAITPID now returns the code: 0..255 for a normal
exit, 256+signal for a violent death. Process args grow from 256
bytes to 4 KiB (cc1 invocations do not fit in 256), with crt1 now
parsing up to 255 argv entries from a static buffer.

libc: new spawn.h with posix_spawn/posix_spawnp over SYS_SPAWN -
libiberty's pex layer has a posix_spawn backend, so GCC's driver works
without fork. argv is joined into the kernel args string (spaces in
arguments rejected; no kernel quoting), envp is not transferred, and
non-empty file actions fail loudly with ENOTSUP until the kernel can
redirect stdio on spawn. waitpid() now decodes real POSIX status and
the sys/wait.h macros distinguish exited from signaled children.

Shell: prints [exit code N] after nonzero exits and [terminated by
signal N] for killed or crashed children.

Verified on the OS: cat on a missing file reports exit code 1; a
window-close (clean exit 0) stays silent as it should.

Co-Authored-By: Claude Fable 5 <[email protected]>

programs/src/shell/exec.cpp

@@ -17,11 +17,33 @@ static bool file_exists(const char* path) {
 
 // ---- Try to spawn an ELF at the given path ----
 
+static void print_exit_code(int code) {
+    if (code == 0) return;
+    char buf[16];
+    int n = 0;
+    if (code >= 256) {
+        montauk::print("[terminated by signal ");
+        code -= 256;
+    } else {
+        montauk::print("[exit code ");
+    }
+    if (code == 0) buf[n++] = '0';
+    while (code > 0 && n < 15) {
+        buf[n++] = (char)('0' + code % 10);
+        code /= 10;
+    }
+    while (n > 0) {
+        char c[2] = { buf[--n], 0 };
+        montauk::print(c);
+    }
+    montauk::print("]\n");
+}
+
 static bool try_exec(const char* path, const char* args) {
     if (!file_exists(path)) return false;
     int pid = montauk::spawn(path, args);
     if (pid < 0) return false;
-    montauk::waitpid(pid);
+    print_exit_code(montauk::waitpid(pid));
     return true;
 }