v0.3.0

Spec v0.2 / proposal 0002: explicit subgraph input/output mapping.

• ExplicitMapping(inputs=..., outputs=...) projection strategy as a built-in alternative to writing a custom ProjectionStrategy.
• New mapping_references_undeclared_field compile error category for inputs/outputs that name fields absent from the relevant schema.
• ProjectionStrategy.validate is an optional duck-typed compile hook — declarative strategies (ExplicitMapping) expose it; imperative custom projections aren't forced to write a no-op.
• Spec submodule pinned at v0.2.0.

See the proposal for design rationale.