Restore benchmarks results page

[Copilot]

playground/benchmarks.html was unintentionally converted into a generic interactive playground by commit d0b6262 ("Standardize all playground pages and add conformance tests"), losing the tsb-vs-pandas results table that aggregates every benchmark.

Changes

• playground/benchmarks.html — Restored to the pre-d0b6262 version: the "Detailed Results" table (function, tsb ms, pandas ms, ratio, faster) populated from benchmarks/results.json, plus the Methodology and About panels.
• tests/playground.test.ts — Added benchmarks.html to NON_PLAYGROUND_PAGES so the playground-conformance suite skips it, alongside the existing index.html exemption. This is what previously pressured the page into the playground shape.

const NON_PLAYGROUND_PAGES = new Set<string>([
  "index.html",
  // The benchmarks page is a results chart (tsb vs pandas), not a playground.
  "benchmarks.html",
]);

Not changed

The earlier removal of the "Visual Comparison" bar chart (commit 1172c14) is left as-is — the issue refers to "the chart … where we listed every single benchmark", which is the results table restored here. Happy to also revert 1172c14 if the bar chart was in fact what was meant.