docs: add roadmap checklist and ADRs

[LimiNode]

Summary

• replace the stale future-plans document with a checked roadmap separating delivered, planned, and intentionally deferred work;
• record the already implemented dedicated executor capability instead of listing it as future work;
• add an ADR template and accepted records for the public header boundary, executor model, immutable registry snapshots, TimeShield compatibility, and benchmark methodology;
• link the roadmap and ADR index from the English/Russian documentation maps and Doxygen input.

Verification

• verified the checklist against current implementation, tests, and documentation;
• git diff --check passed;
• preserved unrelated local/untracked changes.

No generated Doxygen output is included.