fix: report standard medians for even benchmark samples

[cppla]

Summary

• Correct median_mbps and median_duration_ms for even sample counts by averaging the two middle observations.
• Keep the existing p05/p95 nearest-order-statistic convention unchanged; document both conventions and how to recompute older even-count reports from raw samples.
• Avoid overflowing the intermediate sum for large finite, nonnegative measurements.

Regression evidence

• Before the fix, the new test reproduced median([2,10]) = 10 instead of 6, and median([1,2,8,100]) = 8 instead of 5.
• Added empty/single/odd/even/repeated/large-finite cases and explicit unchanged tail-percentile checks.
• Targeted race tests x10, full make check, full make race, formatting and independent review passed.
• Exact head 2f4b67dca7cf744d8f4293f457c8db72a645054f: CI 35811582485 (all 11 jobs including container integration and OCI), CodeQL 35811582479, and Linux netem 35811582480 completed successfully.
• Independent review and automated PR review found no remaining issues; no unresolved review threads.

Scope

Benchmark reporting and documentation only. No transport/controller defaults, wire protocol, dependency, release or tag change. Raw samples and existing experimental binaries/data were not rewritten. This is a statistics correction, not a throughput improvement claim.

Merge verification

• Squash-merged as eecd1d992c46124c3390c0c9ddd918f74d47e011 on main; its tree matches the tested PR head (07a234e7b47d57942b73fffc2ac079d6495e36dc).
• Exact-main CI 35811805160: all 11 jobs passed, including actual container integration, OCI image-index build, race detector and Windows UDP continuity steps.
• Exact-main CodeQL 35811805240 and Linux netem 35811805277 passed; the namespace/netem execution step completed successfully.
• Main working tree is clean. No release or tag created.

[Copilot]

Copilot review overview

🟢 Approval recommended

The reviewed implementation, documentation, and regression coverage have no unresolved blocking issues.

Review effort: Lite
Findings: None

What changed in this PR

Corrects benchmark medians for even-sized samples while preserving existing percentile behavior.

Changes:

• Adds overflow-safe averaging of the two middle observations.
• Documents median and percentile conventions.
• Adds regression coverage for edge cases and unchanged tail percentiles.

File	Summary
docs/​BENCHMARK.md	Documents statistical conventions and legacy report handling.
cmd/​autocar/​bench.go	Implements corrected overflow-safe medians.
cmd/​autocar/​bench_median_test.go	Adds median and percentile regression coverage.

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