Define OpenACC tracing acceptance gates

[krystophny]

PR #354 needs explicit acceptance gates before GPU tracing can merge.

Acceptance:

• Acceleration is off by default unless GPU CI exists.
• CPU path is bitwise unchanged when acceleration is disabled.
• Short-run CPU/GPU tracing agrees deterministically for the accepted configuration.
• Long-run chaotic divergence is checked statistically, not per particle.
• The PR records any CPU performance regression and the benchmark hardware.