debug(fat32): read 包装打印 br 实际值，确认 c.Set 是否写入（types 内 total_read=64 但 read_len<64）

VKernel/Kernel/drivers/fs/fat32/fat32.vp

@@ -107,6 +107,10 @@ def close(fp: types.fileobj | t.CPtr) -> types.FRESULT:
 def read(fp: types.fileobj | t.CPtr, buff: t.CVoid | t.CPtr, btr: t.CUInt32T) -> t.CUInt32T:
 	br: t.CUInt32T = 0
 	fp.read(buff, btr, c.Addr(br))
+	# [RD2-DIAG] 确认 br 是否被 c.Set 写入（types 内 total_read=64 但外部可能收不到）
+	_rd2: t.CArray[t.CChar, 96]
+	viperlib.snprintf(c.Addr(_rd2), 96, "[fat32.vp] read br=%lu\n", br)
+	serial.puts(c.Addr(_rd2))
 	return br
 
 def write(fp: types.fileobj | t.CPtr, buff: t.CConst | t.CVoid | t.CPtr, btw: t.CUInt32T) -> t.CUInt32T: