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/Hal/IDT.cpp

@@ -143,6 +143,16 @@ namespace Hal {
             CrashReport::AddReport(rep);
             Sched::SpawnCrashPad(proc->pid);
 
+            // Record a killed-by-signal exit code (256+sig) so a parent
+            // blocked in SYS_WAITPID can tell a crash from a clean exit.
+            {
+                int sig = 11;                       /* SIGSEGV */
+                if (i == 0x00) sig = 8;             /* #DE -> SIGFPE */
+                else if (i == 0x06) sig = 4;        /* #UD -> SIGILL */
+                else if (i == 0x10 || i == 0x13) sig = 8;  /* x87/SIMD FP */
+                Sched::SetProcessExitCode(proc->pid, 256 + sig);
+            }
+
             Sched::ExitProcess();
             __builtin_unreachable();
         } else {