docs: reorder perf section — cold baseline first, label warm table as 100% cache-hit ceiling

[szibis]

Summary

• Moves the cold proxy results first — the honest floor readers should see before anything else
• Clearly labels the warm cache table as 100% cache-hit results (the ceiling, not the typical case)
• Adds a note that real production performance sits between cold and warm depending on dashboard diversity and refresh interval
• Renames the coalescer section's column header to Proxy P50 (warm) and adds a footnote that first-hit coalescing still pays one backend round-trip
• docs/benchmarks.md was already structured correctly — no changes needed there

Test Plan

• README renders correctly on GitHub
• Cold and warm sections are clearly distinguished
• No benchmark numbers were changed — only ordering and framing

[github-actions]

PR Quality Report

Compared against base branch main.

Coverage and tests

Signal	Base	PR	Delta
Test count	2524	2524	0
Coverage	87.2%	87.2%	0.0% (stable)

Compatibility

Track	Base	PR	Delta
Loki API	100.0%	11/11 (100.0%)	0.0% (stable)
Logs Drilldown	100.0%	17/17 (100.0%)	0.0% (stable)
VictoriaLogs	100.0%	11/11 (100.0%)	0.0% (stable)

Performance smoke

Performance smoke was skipped for this PR because no perf-sensitive paths changed.

State

• Coverage, compatibility, and sampled performance are reported here from the same PR workflow.
• This is a delta report, not a release gate by itself. Required checks still decide merge safety.
• Performance smoke was intentionally skipped because no perf-sensitive paths changed in this PR.
• Delta states use the same noise guards as the quality gate (percent + absolute + low-baseline checks), so report labels match merge-gate behavior.