docs(repo): name the cross-slice read-only import pattern in ARCHITECTURE.md

[w7-mgfcode]

Source

PRP-31 system review:
`.agents/system-reviews/prp-31-mlzoo-d-frontend-registry-explainability-review.md` → "System Improvement Actions" → third action ("Update docs/_base/ARCHITECTURE.md").

Why

The cross-slice read-only import pattern has now been used twice in production code (app/features/explainability/service.py:57 reading ModelRun; app/features/forecasting/service.py with the PRP-31 lazy imports) but is nowhere named in the docs. Without a named pattern in ARCHITECTURE.md, the next maintainer encountering a cross-slice import discussion has to re-derive the rationale from PR history.

The system review marked the new subsection text as DECISIONS LOCKED — the text below should land verbatim.

Change

Add a new subsection "Cross-slice read-only import pattern" to `docs/_base/ARCHITECTURE.md` (verbatim from the system review):

When a feature slice needs to call a service method or read a schema from a different feature slice (e.g., `forecasting/service.py` → `RegistryService`):

• Import at the call site (inside the method), not at module scope, IF any of these are true:

  • The upstream slice's `schemas.py` imports a type from this slice
  • The downstream slice is loaded by `alembic/env.py` at migration time
  • The import would close an SQLAlchemy registry cycle

• Prefer importing the service class over the ORM model — calls go through the public surface, not the persistence layer.

• Document the lazy import with a single-line NOTE comment at the top of the file naming the cycle it breaks.

Existing precedents:

• `app/features/explainability/service.py:57` — read-only `ModelRun` import
• `app/features/forecasting/service.py` — lazy `RegistryService` / `JobService` / `RunStatus` imports inside `get_feature_metadata_for_*` methods (added by PRP-31; required because `RunResponse.model_family` computed_field closes the cycle at alembic cold-boot)

Definition of done

• New subsection lands in `docs/_base/ARCHITECTURE.md`
• PR merged to `dev` with 4 required checks green
• Closes this issue

[w7-mgfcode]

Merged via PR #265 (squash, SHA 37b9885) at 2026-05-20T01:41Z.