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/Sched/Scheduler.hpp

@@ -54,7 +54,7 @@ namespace Sched {
         uint64_t userStackTop;    // User-space stack top
         uint64_t heapNext;        // Simple bump allocator for user heap
         uint32_t readdirCursor;   // Next SYS_READDIR scratch slot
-        char args[256];           // Command-line arguments (set by parent via Spawn)
+        char args[4096];          // Command-line arguments (set by parent via Spawn)
         char user[32];            // Owner user name (inherited from parent on spawn)
         char cwd[256];            // Absolute current working directory
 
@@ -158,6 +158,8 @@ namespace Sched {
 
     // Block the current process until the given PID exits.
     void BlockOnPid(int pid);
+    int LookupExitCode(int pid);
+    void SetProcessExitCode(int pid, int code);
 
     // Block the current process for the given number of milliseconds.
     void BlockForSleep(uint64_t ms);