Draft: Update on Monolith Modularization Strategy

What does this MR do and why?

Adds modular.md — a strategy document for decomposing the GitLab Rails monolith into in-repo Ruby gems, covering five phases:

1. Library extraction — pure-logic gems from lib/gitlab/ (already underway)
2. Layer gems — gitlab-rails-api, -graphql, -sidekiq, -controllers
3. Feature-domain gems — gitlab-<domain>-backend + transport gems per domain
4. gRPC interfaces — formal contracts between gems, enabling microservice graduation
5. Native extensions — Rust rewrites for performance-critical paths

Key design decisions

• Backend gems (-backend) own domain models, services, finders, policies. Transport gems (-api, -graphql, -sidekiq, -controllers) are thin layers included by the corresponding gitlab-rails-* layer gem.
• Four deployment modes per domain: monolith-owned → transport extracted → backend extracted with callbacks → fully standalone.
• Callbacks start as direct Ruby lambdas in initializers, evolve to gRPC. Backend gems must not use monolith AR models directly (allowed during transition as tech debt).
• Transport gems can use monolith AR models directly — they run in-process.
• Internal namespace enforced by RuboCop to prevent cross-gem access to private implementation.
• Phase ordering driven by business impact, not phase number. Phase 1 establishes patterns; Phase 2-5 are prioritized per domain need.
• Glue layer between monolith and domain is expected to be substantial; dependency direction (monolith → backend or backend → monolith) is per-domain.

Relation to existing proposals

References the Modular Monolith and Hexagonal Rails Monolith proposals. This strategy uses gems instead of Packwerk for isolation, and extends scope with gRPC interfaces, native extensions, and data-layer extraction.

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Commits

• Add Modular Monolith

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