Demonstrates the core spike value: the template is now a first-class DSL citizen, so we can cross-validate it against other DSL entities at DSL build time (i.e. inside the Spark transformer pipeline, not in a bolted-on post-compile pass).

We extract @field references from the raw HEEx source and check that each one matches a declared state :field. Unknown references raise a Spark.Error.DslError pointed at the surrounding DSL.