feat(site): split Performance into Passthrough / Translation / Streaming tabs

The single blended Performance table ranked different dialects against each
other (each gateway on its own "Tested on" passthrough), so one sort was not
apples-to-apples. Replace it with three internally-coherent tabs, flat beside
Protocol matrix and Method (Charts folded into Method):

- Passthrough: STRICTLY the openai->openai diagonal, so every gateway ranks on
  the identical dialect with no translation. A gateway that does not serve
  openai ingress in our grid reads n/a rather than borrowing a native-dialect
  number under an OpenAI-passthrough header (e.g. litellm-rust). A gateway that
  serves openai but whose per-cell sweep has not landed falls back to its perf
  suite, which is the same dialect (e.g. bifrost mid-re-run).
- Translation: openai INGRESS fixed (fair input) -> the fastest non-openai
  egress it serves, shown as a Path pill. Only gateways that actually translate
  appear (view-implicit filter).
- Streaming: SSE passthrough, its own stall-gated ceiling.

Each tab carries a one-line caption stating exactly which path it ranks. The
sorted column now gets a faint tint. Matrix cells drop the native title (the
hover popup was doubling up). gen-data emits openai-only best_cell plus a new
translation_cell. Legacy URLs (/results, /charts, #view=) resolve onto the new
tabs. 19 site tests pass.

Author: MattJackson