Re-baseline docs against the ratified unified-arr plan

[thedancingdeveloper]

Summary

• UNIFIED-ARR-PLAN.md becomes the single active roadmap, recording the 2026-08-05 decision resolutions (D5 MariaDB delivery, D8 Indexarr boundary, D9 logical instances/versions) and current 54-issue state.
• PLAN.md, IMPLEMENTATION_PLAN.md, and TODO3.md are reduced to historical pointers — their content predates and is superseded by the canonical plan.
• Topic docs (API-COMPATIBILITY, DATABASE, ARCHITECTURE, CONFIGURATION, DEPLOYMENT, DEVELOPMENT, DOMAIN-MODELS, NOTIFICATIONS, CRATE-GUIDE, clientapp, streaming, phase1-5) updated to distinguish current PostgreSQL-era implementation state from the settled MariaDB 11.4 target state, with cross-references corrected to match the plan's renumbered sections.
• backlog.json entries updated to reflect resolved decisions and corrected section references.

Test plan

• Docs-only change; no build/test impact expected.
• Required status checks pass in CI.