refactor(forecast,registry): move ModelFamily to shared/core to break slice cycle

[w7-mgfcode]

Status: deferred — needs its own PRP.

Origin: CodeRabbit #3270767554 on PR #266.

The flag

`app/features/registry/schemas.py:27` directly imports `ModelFamily` from `app.features.forecasting.schemas`, and `schemas.py:164` lazy-imports `model_family_for` from `app.features.forecasting.feature_metadata`. This violates the vertical-slice invariant (`app/features/X` may NOT import from `app/features/Y`) and is the proximate cause of the existing lazy-import cycle workaround documented in lines 20-27 of that file.

Why deferred

Per project memory `computed-field-cross-slice-cycle` and `data-platform-shared-orm-layer` (in agent memory), the existing fix (lazy in-method import + a NOTE comment naming the cycle) is the documented pattern. The proper resolution — relocating `ModelFamily` + `model_family_for` to a neutral module — is a non-trivial refactor that touches:

• Both slices' schemas + service imports
• The newly-added cross-slice import pattern doc in `docs/_base/ARCHITECTURE.md` (PR docs(repo): name cross-slice read-only import pattern (#264) #265)
• Likely an ADR if the chosen module isn't an obvious fit

That belongs behind a PRP, not a hot-fix commit on a release-prep PR.

Proposed scope (when a PRP lands)

• Create `app/shared/model_taxonomy.py` (or extend `app/shared/models.py`) with `ModelFamily` + `model_family_for(model_type) -> ModelFamily`.
• Re-export from `app/features/forecasting/{schemas,feature_metadata}.py` to preserve the public API of those modules.
• Update `app/features/registry/schemas.py` to import from the shared module.
• Drop the lazy-import workaround in `app/features/forecasting/service.py` get_feature_metadata_for_* methods if the cycle is fully broken.
• Update the ARCHITECTURE.md cross-slice pattern note to add this case as a "broken cycle" example.

Sibling case

CodeRabbit also keeps flagging `app/features/data_platform/models.py` cross-slice imports for the same reason — that one is the de-facto shared ORM layer and is tracked separately (HANDOFF.md Open Questions).

Refs: PR #266