Refresh charts: 5-round eval-results.svg + label kdense on the frontier

The main README's headline chart (eval-results.svg) still showed only rounds
1-3 and claimed "first in all three rounds," contradicting the round-5 section.
Replace it with an honest 5-round summary: ours' score held near 8-9 throughout,
first vs baseline/rivals in rounds 1-4, #3 of 19 in the round-5 open field.

Also label the new top point (kdense) on the quality-vs-cost frontier banner,
and update the chart's README caption.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>