docs: fix architecture diagram text alignment

[zevorn]

Summary

• Fix centering of all label text in the ASCII architecture diagram
• Correct osal/claw_os.h (OSAL API) double-space typo
• Center RT-Thread and QEMU vexpress-a9 in their split-cell columns
• Applied to both README.md and README_zh.md

Before / After

Before (misaligned):

|            (one skill composes multiple tools)               |
|                     tools (Tool Use)                         |
|                   drivers (Hardware BSP)                     |
|               osal/claw_os.h  (OSAL API)                     |
| FreeRTOS (IDF)    |             RT-Thread                    |
| ESP32-C3 / S3     |  QEMU vexpress-a9                        |

After (centered):

|             (one skill composes multiple tools)              |
|                       tools (Tool Use)                       |
|                    drivers (Hardware BSP)                    |
|                  osal/claw_os.h (OSAL API)                   |
| FreeRTOS (IDF)    |                RT-Thread                 |
| ESP32-C3 / S3     |             QEMU vexpress-a9             |