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]>

kernel/src/Api/Process.hpp

@@ -20,7 +20,7 @@
 
 namespace montauk::abi {
     static void Sys_Exit(int exitCode) {
-        (void)exitCode;
+        Sched::SetProcessExitCode(Sched::GetCurrentPid(), exitCode & 0xFF);
         Sched::ExitProcess();
     }
 
@@ -36,8 +36,11 @@ namespace montauk::abi {
         return Sched::GetCurrentPid();
     }
 
-    static void Sys_WaitPid(int pid) {
+    // Returns 0..255 for a normal exit, 256+signal for killed/crashed,
+    // 0 when the pid is unknown or its ledger entry has been evicted.
+    static int Sys_WaitPid(int pid) {
         Sched::BlockOnPid(pid);
+        return Sched::LookupExitCode(pid);
     }
 
     static int Sys_Spawn(const char* path, const char* args) {