add semantic concept contracts, catalog correlation, and rules

[matthewpeterkort]

Loom Explorer Builder stack

Layer 4 of 6. Merge strictly in order: #17 → #18 → #19 → #20 → #21 → #22.

Previous: #19
Next: #21

What changed

• freezes the Explorer builder semantic-concept v2 contract
• adds correlated semantic observations to the catalog
• adds the semantic concept registry and rule engine

Why

The builder needs a stable semantic layer above raw FHIR paths so frontend selections represent clinical concepts instead of backend storage details.

Impact

Loom can derive and validate semantic concepts from catalog observations using explicit, testable rules.

Validation

The complete stack tip (b281499c) passes go test ./.... This layer also runs independent GitHub CI, generated-code checks, and a server image build.

[matthewpeterkort]

Retaining this PR as warm-storage dependency/reference material for the semantic catalog work in #21. It is not planned for merge now and will need to be selectively ported onto current #17 when revived. The head branch is preserved.