docs(repo): name cross-slice read-only import pattern (#264)

[w7-mgfcode]

Summary

• New subsection "Cross-slice read-only import pattern" in docs/_base/ARCHITECTURE.md, naming a convention now used twice in production (explainability/service.py:57, forecasting/service.py per PRP-31).
• Text is verbatim from the PRP-31 system review's DECISIONS LOCKED block.

Closes #264.

Why

The pattern has shipped twice but is nowhere documented. Without naming it in ARCHITECTURE.md, the next maintainer encountering a cross-slice import discussion has to re-derive the rationale from PR history. Source: .agents/system-reviews/prp-31-mlzoo-d-frontend-registry-explainability-review.md § "System Improvement Actions" → third action.

Validation gates (local)

• uv run ruff check . && uv run ruff format --check . ✅
• uv run mypy app/ ✅ (276 source files)
• uv run pyright app/ ✅ (0 errors, 68 pre-existing warnings)
• uv run pytest -m "not integration" ✅ (1423 passed)

Test plan

• CI checks green on dev (Lint & Format, Type Check, Test, Migration Check)
• One reviewer
• Merge via the web UI

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