feat(validation): configure the validator by rule identity

Move the validator toward the validation-rules idiom's identity-based
configuration. withoutValidating(_:) removes a rule by its description, the
removal identity the idiom uses, so descriptions must be unique and stable.
validationDescriptions lists the active rules in application order.

A configuration-pin test asserts the exact descriptions of the blank and
default validators and that the default descriptions are unique, so adding,
removing, or rewording a rule is a deliberate change that a test catches.
Both additions are non-breaking; the type-directed Validation<Subject>
dispatch and the error-rendering change remain the larger migration steps.

CHANGELOG.md

@@ -9,6 +9,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- The validator is now configurable by rule identity, per the validation-rules idiom:
+  `withoutValidating(_ description:)` removes a rule by its description (the removal identity, so
+  descriptions must be unique and stable), and `validationDescriptions` lists the active rules in
+  application order. A configuration-pin test asserts the exact descriptions of the blank and
+  default validators, so adding, removing, or rewording a rule is a deliberate change.
 - A validation-coverage gate (`scripts/check-validation-coverage.sh`, wired into
   `scripts/check-all.sh` and CI) enforcing the validation-rules discipline that every public type is
   either validated or excluded. It enumerates the public surface from the compiler symbol graph (the

Sources/Validation/Validator.swift

@@ -63,6 +63,21 @@ public extension PureYAML.Validation {
             copy.rules.append(rule)
             return copy
         }
+
+        /// Removes every rule whose description matches, the identity the validation idiom uses for
+        /// removal. Descriptions are therefore required to be unique and stable across the rule set.
+        public func withoutValidating(_ description: String) -> Self {
+            var copy = self
+            copy.rules.removeAll { $0.description == description }
+            return copy
+        }
+
+        /// The descriptions of the active rules, in application order: the fixed point a
+        /// configuration-pin test asserts against, so adding, removing, or rewording a rule changes
+        /// it deliberately.
+        public var validationDescriptions: [String] {
+            rules.map(\.description)
+        }
     }
 }
 

Tests/ValidationConfigurationTests.swift

@@ -0,0 +1,39 @@
+@testable import PureYAML
+import Testing
+
+@Suite("Validator configuration")
+struct ValidationConfigurationTests {
+    private var duplicateKeys: PureYAML.Model.Value {
+        .mapping(.init([
+            .init(key: "a", value: .int(1)),
+            .init(key: "a", value: .int(2)),
+        ]))
+    }
+
+    @Test("The blank validator has no rules; the default set is pinned by description")
+    func test_configurationDescriptionsArePinned() {
+        #expect(PureYAML.Validation.Validator.blank.validationDescriptions == [])
+        #expect(PureYAML.Validation.Validator().validationDescriptions == ["Mapping keys are unique"])
+    }
+
+    @Test("Default rule descriptions are unique, the identity withoutValidating removes by")
+    func test_defaultDescriptionsAreUnique() {
+        let descriptions = PureYAML.Validation.Validator().validationDescriptions
+        #expect(Set(descriptions).count == descriptions.count)
+    }
+
+    @Test("withoutValidating removes a rule by its description")
+    func test_withoutValidatingRemovesByDescription() throws {
+        let validator = PureYAML.Validation.Validator().withoutValidating("Mapping keys are unique")
+        #expect(validator.validationDescriptions == [])
+        // With the rule removed, a document with duplicate keys validates clean.
+        _ = try validator.validate(duplicateKeys)
+    }
+
+    @Test("The default validator still catches what its removable rule guards")
+    func test_defaultStillValidates() {
+        #expect(throws: PureYAML.Validation.Issue.Collection.self) {
+            try PureYAML.Validation.Validator().validate(duplicateKeys)
+        }
+    }
+}