fix(parsing): keep a root node that begins on the document-marker line

A document-start marker scan skipped the rest of the '---' line
unconditionally, so a root node written on that same line was discarded:
a block-scalar header ('--- >' / '--- |') was dropped and its body
mis-read as a plain scalar (folded to spaces, no trailing newline), and a
same-line plain or flow node was lost entirely. The document start now
skips only trailing spaces and leaves any real same-line content for the
parser; an empty rest-of-line or a comment is still consumed, and a node
on the line below the marker is unaffected. Document end ('...') still
takes no same-line node.

Measured against the official YAML test-suite, value-level conformance
rises from 130/170 (76.5%) to 135/169 (79.9%) with no regression.
Parse-level is unchanged: these documents already parsed, just to the
wrong tree, which only the value-level metric exposes.

CHANGELOG.md

@@ -23,6 +23,16 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- A root node that begins on the document-marker line (`--- >`, `--- |`, `--- foo`,
+  `--- {a: 1}`) is no longer discarded. The marker scanner skipped the rest of the
+  `---` line unconditionally, so a block-scalar header was dropped and its body was
+  mis-read as a plain scalar (folded to spaces, no trailing newline) and a same-line
+  plain or flow node was lost entirely. The document start now keeps any real
+  same-line content for the parser, skipping only trailing spaces or a comment.
+  Lifts official test-suite value-level conformance from 130/170 (76.5%) to
+  135/169 (79.9%), with no regression; parse-level is unchanged because these
+  documents already parsed, just to the wrong tree, which only the value-level
+  metric could see.
 - A multi-line plain scalar whose value begins on its own line under a mapping key
   now folds its continuation lines instead of stopping after the first. The
   continuation check compared the next line's indent against the value's own pushed

Sources/Parsing/Scanner.Directives.swift

@@ -38,6 +38,19 @@ extension PureYAML.Parsing.Scanner {
         state.reader.advance()
         state.reader.advance()
         state.append(kind, mark: start, endMark: state.reader.mark)
+        // The document's root node may begin on the marker's own line (e.g. `--- >`, `--- foo`,
+        // `--- {a: 1}`, `--- !!str x`). After a document start, skip only the spaces and leave any
+        // real content for the main loop to scan; an empty rest-of-line or a comment is consumed.
+        // A document end (`...`) takes no same-line node, so its line is always skipped.
+        if case .documentStart = kind {
+            while state.reader.peek() == " " {
+                state.reader.advance()
+            }
+            if let next = state.reader.peek(), !isLineBreak(next), next != "#" {
+                state.isAtLineStart = false
+                return
+            }
+        }
         skipLine(&state)
     }
 

Tests/DocumentMarkerNodeTests.swift

@@ -0,0 +1,20 @@
+@testable import PureYAML
+import Testing
+
+@Suite("Document marker root nodes")
+struct DocumentMarkerNodeTests {
+    @Test("A block scalar header on the document-marker line is recognized, not discarded")
+    func test_blockScalarHeaderOnDocumentMarkerLine() throws {
+        // `--- >` / `--- |`: the marker scanner used to skip the rest of the line, dropping the
+        // block header so the body was mis-read as a plain scalar (folded, no trailing newline).
+        #expect(try PureYAML.parse("--- >\n  a\n  b\n") == .string("a b\n"))
+        #expect(try PureYAML.parse("--- |\n  a\n  b\n") == .string("a\nb\n"))
+    }
+
+    @Test("A scalar root node may begin on the document-marker line")
+    func test_scalarRootNodeOnDocumentMarkerLine() throws {
+        #expect(try PureYAML.parse("--- folded text\n") == .string("folded text"))
+        // A node still on its own line below the marker keeps working.
+        #expect(try PureYAML.parse("---\nplain text\n") == .string("plain text"))
+    }
+}