Store timestamps in nanosecond precision

We currently store timestamps in float format while most interfaces and log formats use some integer representation. Converting these to floating point is a lossy operation.

My proposal is to add an additional `timestamp_ns` argument and attribute to the Message class. On initialization, if the `timestamp` argument is given it would be converted to ns. The `.timestamp` attribute can be dynamically calculated on get and set.

It would be fairly trivial to update the interfaces and log formats to use ns timestamps instead.

先定位 Message 類別，並閱讀其 timestamp 的初始化、儲存和公開方式。接著追蹤提案中提到的介面和日誌格式，以識別每個 timestamp 邊界。當奈秒值保持無損，且現有的 timestamp 存取在這些介面和格式之間持續一致地運作時，即表示完成。

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