Refresh README and docs for the current release

[FumingPower3925]

The README still headlines v1.5.7, the hero caption says v1.5.6 bench data, and the "fastest in N of M" line is hardcoded and stale versus the published dashboard.

• Update the What's-new section + version highlight to the current release
• Fix the hero caption to the current bench data
• Re-derive the win-rate headline from the published results instead of hardcoding it
• docs/README results-tree note reflects the current version range

[FumingPower3925]

Text refresh is in #454 (what's-new -> v1.5.8, headline corrected to 52 servers / 26 of 29, arm64 note added). Still open under this issue: regenerate the hero chart .github/benchmark.png from v1.5.8 data (it's v1.5.6-era today; perf is flat across the correctness releases so the visual barely moves, but the provenance should match), and the docs/README results-tree note.