Add Ruby integration plan and reference it from README

[blackopsrepl]

Motivation

• Provide a formal plan for Ruby-first modeling integration that preserves SolverForge's zero-erasure performance invariants and documents architecture, goals, phases, and acceptance criteria.

Description

• Add docs/ruby-integration-plan.md containing goals, non-negotiable performance invariants, architecture layers, validation policy, implementation phases, observability gates, and acceptance criteria, and update README.md to reference the new document in the Documentation Map.

Testing

• Documentation-only change; no automated unit or integration tests were run as part of this PR.

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Codex Task

[blackopsrepl]

Closing this PR because the Ruby integration work is now continuing in the standalone solverforge-ruby repository and is no longer coupled to the SolverForge monorepo.