lower semantic concept selections into dataframe recipes

[matthewpeterkort]

Loom Explorer Builder stack

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

Previous: #21
Next: merge the completed stack and deploy the resulting Loom image.

What changed

• lowers semantic concept selections into recipe fields and traversals
• resolves catalog-backed selections during recipe compilation
• integrates semantic catalog discovery with project recipe revisions
• adds end-to-end mapping, lowering, and integration coverage

Why

Catalog discovery alone does not affect execution. This layer connects frontend semantic selections to executable, versioned dataframe recipes.

Impact

A builder frontend can select semantic concepts and have Loom produce and execute the corresponding recipe representation.

Validation

This is the complete integration tip (b281499c) and passes go test ./.... GitHub CI also runs generated-code checks and builds the server image.

[matthewpeterkort]

Retaining this as warm-storage reference material for the recipe integration/column-selection work in #24. It is not planned for merge now and will need to be selectively ported onto current #17. The head branch is preserved.