feat(regex): make callback mutations transactional

[fglock]

Summary

• make ordinary scalar, array, and hash mutations from executable regex callbacks transactional at the matcher-attempt boundary
• restore callback-visible state when the overall match fails or a callback throws, while retaining mutations when the match succeeds
• hide internal callback IDs from regex stringification without breaking embedded callback remapping
• update the Phase 36 plan to record the recovered callback baseline and remaining scope/caller/control-flow work

Compatibility

• perl5_t/t/re/rxcode.t: 42/42 (previously 39/42)
• perl5_t/t/re/reg_eval_scope.t: 22/49, unchanged and documented as the next Stage 36.4 slice
• failed overall matches roll back ordinary callback mutations
• successful matches retain mutations from both abandoned and selected alternatives, matching system Perl

Validation

• make — PASS
• system Perl: callback_mutation_rollback.t — 9/9
• JVM and interpreter: callback_mutation_rollback.t — 9/9 each
• system Perl: callback_stringification.t — 2/2
• JVM and interpreter: callback_stringification.t — 2/2 each
• focused upstream runner: rxcode.t 42/42; reg_eval_scope.t 22/49; no timeout or incomplete file