fix(sched): 通过 rdi 传递用户线程入口参数

用户线程入口的第一个形参在 rdi，但 drop_to_user_mode 这条路径从未设置过 rdi，
thread_create(fn, arg) 的 arg 在用户侧一直是垃圾值。iretq 只恢复
RIP/CS/RFLAGS/RSP/SS，因此在 iretq 前显式 mov rdi 即可。

VKernel/Kernel/sched/process.vp

@@ -256,7 +256,7 @@ def get_pid(tid: int) -> int:
                 return p.pid
     return -1
 
-def drop_to_user_mode(entry: rawptr, user_rsp: uint64, kern_rsp: uint64):
+def drop_to_user_mode(entry: rawptr, user_rsp: uint64, kern_rsp: uint64, arg: rawptr = None):
     # NOTE: Do NOT swapgs here. This OS uses reverse GS convention:
     #   GS_BASE = 0 (kernel & user), KERNEL_GS_BASE = &_per_cpu
     #   syscall/ISR entry does swapgs to get GS_BASE = &_per_cpu
@@ -270,6 +270,11 @@ def drop_to_user_mode(entry: rawptr, user_rsp: uint64, kern_rsp: uint64):
     # the stack pointer becomes garbage, and even the page-fault frame cannot be pushed
     # => triple fault with zero output (not even the ISR entry probe can fire).
     # Swapgs only when GS_BASE != 0 (idempotent; same idiom as the handler's GS restore).
+    # 【修复】用户线程入口的第一个形参在 rdi —— 这条路径原来**从没设过 rdi**，于是
+    # `thread_create(fn, arg)` 的用户参数在用户侧永远是垃圾值。iretq 只恢复
+    # RIP/CS/RFLAGS/RSP/SS，其余通用寄存器原样保留，所以在这里设 rdi 即可传递。
+    arg_val: uint64 = 0
+    if arg is not None: arg_val = uint64(arg)
     __asm__(f"""cli
             mov rsp, {asminp(kern_rsp, ASM_DESCR.REG_ANY)}
             mov ecx, 0xC0000101
@@ -280,6 +285,7 @@ def drop_to_user_mode(entry: rawptr, user_rsp: uint64, kern_rsp: uint64):
             jz 1f
             swapgs
             1:
+            mov rdi, {asminp(arg_val, ASM_DESCR.REG_ANY)}
             mov rdx, {asminp(user_rsp, ASM_DESCR.REG_ANY)}
             mov rcx, {asminp(entry, ASM_DESCR.REG_ANY)}
             mov r11, 0x202

VKernel/Kernel/sched/sched.vp

@@ -438,7 +438,7 @@ def _user_thread_entry(arg: rawptr) -> int:
     proc.switch_pid(ctx.pid)
     gdt.set_tss_rsp0(ctx.kern_stack)
     cpu.set_kernel_rsp0(ctx.kern_stack)
-    proc.drop_to_user_mode(ctx.entry, ctx.user_stack_entry_rsp, ctx.kern_stack)
+    proc.drop_to_user_mode(ctx.entry, ctx.user_stack_entry_rsp, ctx.kern_stack, ctx.arg)
     return 0
 
 def thread_current() -> int: