Add “entity / grain” as a first-class concept

[anirudhagarwal-atlan]

Right now the spec has:

• dataset keys
• relationships
• metrics as expressions

But it’s missing the concept that makes semantics robust:

• what grain is this metric defined at?
• what joins are allowed without changing meaning?
• which dimensions are valid for this metric?

Recommendation
Add:

• datasets[].kind: FACT|DIMENSION (or similar)
• datasets[].entities (or grain)
• metrics[].grain and/or metrics[].base_dataset

Even a minimal “base_dataset + grain entity” prevents a ton of wrong queries by downstream tools/agents.