Introduce the core of the validation-rules idiom: a Check<Subject> with a
positive description, a when predicate, and a Bool or multi-error check
that fires only on values of its Subject type wherever they occur. AnyCheck
erases the subject and dispatches by runtime type, guarding an optional or
a wrong type from matching. The validator gains a checks list, a
validating(_:) overload, and a traversal that offers each node the values
it may specialise on. BuiltinCheck.mappingKeysAreUnique is the
type-directed counterpart of the legacy rule.
The machinery has its own negative tests (wrong type, optional, false
predicate yield nothing; same type twice yields two errors) and the builtin
ships a failing and a near-miss test. The coverage gate catches the five
new public types, which are classified as validation machinery. The default
validator is unchanged, so this is additive; retiring the legacy Rule,
moving the default onto the check, the combinator algebra, the error
rendering, and de-duplicating the Tagged validator are the remaining steps.
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.
Split the six files that declared multiple sibling types under one
namespace extension so each file declares exactly one top-level type:
the Tagged node component types (Node, NodeKind, Scalar, Sequence,
Mapping, Pair), the validation report types (Report, Source,
SourceReport, BatchReport), DiagnosticKind, TokenKind, ScannerFlowContext
kind, and the event-stream composer helpers. Public type names are
unchanged; the shared report-rendering helpers move from file-private to
internal so the split report types still reach them.
Make check-namespacing.sh namespace-aware: the previous column-anchored
scan was blind to types declared inside indented extension blocks, so it
passed while the rule was violated. It now counts direct members of a
PureYAML namespace extension and ignores nested types and private helpers.
Add structured validation diagnostics, JSON and Yams comparison gates, and lazy token/event parsing so production behavior is measured without retaining full intermediate parser arrays.
