Pass argv to processes on a SysV entry stack; grow the user stack to 32 KiB

Processes now start with C-compatible arguments: the kernel builds the
System V AMD64 entry block (argc, argv, empty envp, auxiliary vector)
at the top of the stack, argv[0] is the path or initial-ramdisk name
the process was spawned as, and system_spawn carries an optional
NUL-separated blob that becomes argv[1..]. The runtime parses the block
(runtime.argumentCount/argument) and its spawn wrappers pass arguments
through. The name is also recorded on the task, so a fault report says
which binary died, not just its id.

The user stack grows from one page to eight (32 KiB,
parameters.user_stack_pages), with the page below left unmapped as a
guard so an overflow faults into a clean process kill rather than
corrupting the image. Task.name_buffer is zero-initialised, not
undefined: an undefined default is materialised as a 0xAA fill that
moved the static task pool out of .bss and made the whole kernel ~7x
slower under QEMU TCG (caught by the affinity test).

Proven end to end by the new args test: args-echo respawns itself with
arguments via the syscall blob, burns more stack than one page could
hold, and echoes its argv intact. Full suite: 44/44.

system/parameters.zig

@@ -22,6 +22,12 @@ pub const maximum_tasks = 16;
 /// Each task's kernel stack (also each AP's bring-up stack), in bytes.
 pub const kernel_stack_size = 16 * 1024;
 
+/// Each user process's stack, in pages (32 KiB). Mapped just below a fixed top;
+/// the System V entry block (argc/argv) occupies the top of the highest page, and
+/// the page below the mapping is left unmapped as a guard, so an overflow faults
+/// (killing only that process) instead of silently corrupting the image.
+pub const user_stack_pages = 8;
+
 /// Each core's IST (double-fault) stack, in bytes. The BSP's is static; an AP's is
 /// heap-allocated at bring-up.
 pub const ist_stack_size = 16 * 1024;