docs(regex): replace Phase 36 with forward-only plan

[fglock]

Summary

• replace the historical Phase 36 transcript with one forward-only execution plan
• define vendored Joni as the sole final matcher and retain only Perl source policy in the frontend
• specify ordinary-pattern, Unicode/fold, runtime-source, preprocessing/import cleanup, documentation, performance, platform, and PR 958 parity gates
• add a proposal-only, post-Phase-36 RFC for an embeddable Perl-compatible JVM regex library

Validation

• exact-head make at e531cee3b: all 17 tasks green in 4m40s (/tmp/make-phase36-plan-master-e531cee3b.log)
• branch changes only dev/design/phase36-regex-parity.md and dev/design/perl-regex-library-rfc.md
• final files are byte-identical to the previously reviewed documentation content
• the plan contains no dated implementation history or PR transcript
• every phase has an explicit exit condition and the final PR 958 comparison remains mandatory

Target correction

The earlier documentation PR was accidentally based on and merged into a
temporary sync branch. This PR contains only the intended two-file documentation
result on current master.

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