docs: make the performance boundary readable

[tsenart]

What changed

• Put the 33/33 arena result beside the 2/5 Rebar result and state the claim boundary plainly.
• Explain how the byte sieve, compiled plan, AVX-512 kernels, and exact confirmation work together.
• Link the verified targeted result, its open PR, and the live raw-byte Case.
• Update the Rebar audit so stopped experiments are not described as open work.

Verification

• go test ./...
• python3 scripts/verify_benchmarkbar_test.py
• python3 scripts/verify_throughput_test.py
• ./scripts/check-baseline-isolation.sh
• git diff --check