expose the authenticated semantic concept catalog

[matthewpeterkort]

Loom Explorer Builder stack

Layer 5 of 6. Merge strictly in order: #17 → #18 → #19 → #20 → #21 → #22.

Previous: #20
Next: #22

What changed

• exposes semantic concept catalog queries through GraphQL
• scopes catalog access through the existing authorization boundary
• adds resolver mappings, generated models, and API tests

Why

The semantic core must be available through the public builder API before a frontend can discover and select concepts.

Impact

Authorized frontend clients can retrieve semantic concepts for the active project graph.

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 as one of the two warm-storage ideas. The semantic catalog API depends on the semantic core represented by #20, but this stacked branch is not planned for merge as-is. Future revival should start from current #17 and selectively port the API/contracts. The head branch is preserved.