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.

library/runtime/runtime.zig

@@ -26,5 +26,10 @@ pub const dma = @import("dma.zig");
 /// Re-exported so a user binary can `pub const panic = runtime.panic;`.
 pub const panic = start.panic;
 
+/// Process arguments (argc/argv, parsed from the kernel-built entry stack):
+/// `argument(0)` is the path or name this binary was spawned as.
+pub const argumentCount = start.argumentCount;
+pub const argument = start.argument;
+
 /// The heap as a `std.mem.Allocator`, for Zig `std` containers in user code.
 pub const allocator = heap.allocator;