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.
The buffered and streaming event composers each carried an identical
copy of the merge-key (<<) override logic and its invalid-value error.
Extract both into one internal Parsing.MergeResolution helper so the
precedence rules (local keys win, earlier sources win, inherited then
local order) have a single source of truth. No behavior change; all
tests pass.
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.
