Make validation cancellation authoritative

docs/internal/sources.md

@@ -130,6 +130,16 @@ their paths once and enforce the configured source boundary. The
 provider-facing structured-output metadata uses the root document captured by
 the same plan.
 
+Schema preparation and execution remain synchronous. Promptkit checks
+cancellation before and after source resolution, JSON decoding, compilation,
+and validation, and between bounded schema-read chunks. Once cancellation is
+observed it returns the context error without publishing a partial plan or
+validation result, even when a compiler or validator has just returned a
+different error or a successful result. An `fs.FS` method or JSON Schema
+dependency call already in progress cannot be preempted; Promptkit waits for
+that call to return and then gives cancellation precedence. Validation does
+not detach dependency work into background goroutines.
+
 `Prepare` discards its validation plan after returning metadata. `Run` retains
 its plan for initial and repaired-output validation, then discards it with the
 operation. `PrepareExecution` retains the plan in its private frozen payload;
@@ -139,6 +149,7 @@ performs fresh source resolution and preparation.
 
 The [validator tests](../../internal/validate/standard_validator_test.go) own
 basic, JSON, JSON Schema, source resolution, schema loading, compilation,
-frozen-reference behavior, and content-failure behavior. Prepared execution
+frozen-reference behavior, content-failure behavior, and the synchronous
+cancellation boundary. Prepared execution
 orchestration is owned by the
 [use-case tests](../../internal/usecase/prepared_execution_test.go).