Add incremental plan and benchmark helper for join key pre-filtering RFC

[nicepopo86-lang]

Summary

This PR adds a concrete incremental implementation aid for RFC #5093:

1. docs/dev/join-key-prefiltering-rfc-notes.md
   • narrows the optimization into safe rollout phases
   • defines guardrails, config knobs, and validation matrix
2. docs/dev/queries/join-key-prefiltering.ppl
   • reusable query fixture for this join shape
3. scripts/benchmark_join_prefilter.sh
   • lightweight benchmark helper for /_plugins/_ppl
   • supports hyperfine when available, curl fallback otherwise

Why this helps

The issue is an RFC/performance optimization topic; this PR converts it into a reviewable and testable next step without introducing risky planner changes in one shot.

Validation

• bash -n scripts/benchmark_join_prefilter.sh
• script is executable and documented

Refs #5093

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