docs(libfabric): add targeted performance review for libfabric plugin

[kpjeeja]

Motivation

• Provide a focused, actionable performance review of the Libfabric backend to capture hotspots and prioritised optimizations before making behavioral changes.

Description

• Add src/plugins/libfabric/PERFORMANCE_REVIEW.md with concrete recommendations covering progress-loop backoff, batched CQ draining, hot-path logging reduction, notification-key design and map choice, control-path buffer reuse, adaptive striping, active-rail progression lock/copy overhead, and a suggested benchmarking/rollout plan; no runtime code changes were made.

Testing

• Ran repository scans and file inspections (e.g., rg -n "libfabric|fabric|fi_", nl -ba/sed -n on relevant files) to validate findings and context, and committed the new documentation with git (all commands completed successfully).

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Codex Task