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/Syscall.cpp

@@ -47,7 +47,8 @@ namespace montauk::abi {
 
     static constexpr uint64_t kMaxPrintableStringBytes = 4096;
     static constexpr uint64_t kMaxPathBytes = 256;
-    static constexpr uint64_t kMaxArgsBytes = 256;
+    // Large enough for compiler driver command lines (cc1/ld invocations).
+    static constexpr uint64_t kMaxArgsBytes = 4096;
     static constexpr uint64_t kMaxWindowTitleBytes = 256;
     static constexpr uint64_t kMaxHostnameBytes = 256;
     static constexpr uint64_t kMaxUserNameBytes = 32;
@@ -140,8 +141,7 @@ namespace montauk::abi {
                 return (int64_t)Sys_Spawn((const char*)frame->arg1,
                                           UserMemory::IsUserPtr(frame->arg2) ? (const char*)frame->arg2 : nullptr);
             case SYS_WAITPID:
-                Sys_WaitPid((int)frame->arg1);
-                return 0;
+                return Sys_WaitPid((int)frame->arg1);
             case SYS_FBINFO:
                 if (!UserMemory::Writable<FbInfo>(frame->arg1)) return -1;
                 Sys_FbInfo((FbInfo*)frame->arg1);