brand(tritrpc): add logo asset and capture landscape positioning

[mdheller]

Summary

This PR captures the remaining repo-local prep needed before opening an upstream ecosystem/landscape submission.

It adds:

• assets/tritrpc.svg
• LANDSCAPE.md

Why

TriTRPC already has the protocol surface needed for external positioning as an Agent Protocols project:

• stable TritRPC v1 specification
• canonical encoding contract
• cross-language parity across Rust and Go
• strict fixture verification
• authenticated envelope framing

The missing blocker on the repo side was simply having a reusable logo asset and one in-repo note that captures the intended category, tag, one-line description, and listing metadata.

Scope

This PR does not change the protocol, fixtures, or implementation.
It only adds lightweight brand/submission-prep artifacts so the external listing work is captured in the repo itself.

Intended external positioning

• Category: Agent Protocols
• Summary tag: Open Source
• One-line description: Deterministic, authenticated RPC protocol with canonical fixtures and cross-language parity for agent interoperability.