docs: design executable regex callbacks

[fglock]

Summary

• add a focused design for executable regex callbacks
• define the structured callback template and generic Joni callout architecture
• specify Perl backtracking, dynamic-local, provisional-capture, caching, and safety requirements
• lay out differential, core-suite, and CPAN policy-removal gates across seven phases
• link the older broad Joni migration sketch to the focused callback design

Key decisions

• keep ordinary regexes on the existing fast path
• preserve callback bodies as lexical RuntimeCode closures instead of flattening them into pattern strings
• extend Joni with runtime-neutral callout opcodes rather than adding regex-preprocessor approximations
• restore dynamically localized state on backtracking without rolling back ordinary callback side effects
• require (??{ ... }) nested alternatives to participate in outer backtracking before enabling the feature

Verification

• git diff --check
• full make (BUILD SUCCESSFUL, 5m42s)

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Co-Authored-By: Codex codex@openai.com