[Draft] Integrate MTGS renderer into Alpasim

[WCJ-BERT]

Description

This PR is a draft of our ongoing integration work. It introduces:

• Trajdata support: adds an initial data-loading / interface layer to run the pipeline on Trajdata.

• Initial MTGS integration: wires in a preliminary MTGS-based rendering path to enable end-to-end testing within the existing framework.

We’re aware that the current iteration includes a proto change that is potentially disruptive to the existing architecture. This was done to unblock the prototype, but we agree it’s not ideal and we will redesign it to restore compatibility.

Since this is an early integration, we’d really appreciate maintainers’ help to:

• sanity-check the integration direction (TrajData + MTGS wiring),

• highlight any architectural or API issues we may have missed,

• point out any additional breaking changes or downstream impacts beyond the proto update.

We will follow up with cleanup, compatibility fixes, and more complete tests once we align on the preferred design.

TODO

1. Remove ego pose from the existing proto.
2. Improve reactive traffic handling in the untested (non-test) path.
3. Move MTGS-related services and server code into the render/ directory.