fix(parsing): classify the keep block-scalar chomping indicator

The scanner consumed the `+` indicator of a `|+` / `>+` block scalar but
mapped it to nothing, so keep was silently treated as clip, and a scanner
test baked that in. Add a `keep` case to BlockScalarChomping, recognize
`+` in the header scanner, and correct the test. Full keep value semantics
(preserving trailing blank lines) depend on scanner blank-line capture and
are tracked in #57; this commit fixes the token classification only.

CHANGELOG.md

@@ -22,6 +22,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   win, earlier sources win, inherited-then-local ordering) now have one source
   of truth. No behavior change.
 
+### Fixed
+
+- Classify the block scalar keep chomping indicator (`|+` / `>+`) as a distinct
+  `keep` chomping instead of silently treating it as clip. Full keep semantics
+  (preserving trailing blank lines) still depend on scanner blank-line capture,
+  tracked in #57.
+
 ### Documentation
 
 - Correct `SECURITY.md`: fix the reporting contact address and replace the

Sources/Parsing/BlockScalarChomping.swift

@@ -2,6 +2,7 @@ extension PureYAML.Parsing {
     enum BlockScalarChomping: String, Equatable, CustomStringConvertible {
         case clip
         case strip
+        case keep
 
         var description: String {
             rawValue

Sources/Parsing/Scanner.swift

@@ -301,6 +301,8 @@ extension PureYAML.Parsing.Scanner {
         while let character = state.reader.peek(), isBlockScalarHeaderCharacter(character) {
             if character == "-" {
                 chomping = .strip
+            } else if character == "+" {
+                chomping = .keep
             } else if character.isNumber {
                 indentationIndicator = Int(String(character))
             }

Sources/Parsing/TokenEventParser.Block.swift

@@ -249,7 +249,13 @@ extension PureYAML.Parsing.TokenEventParser {
         switch chomping {
         case .strip:
             return content
-        case .clip:
+        case .clip, .keep:
+            // Clip and keep currently coincide: both append the single final
+            // line break. They diverge only over trailing blank lines, which the
+            // scanner does not yet capture into `lines`, so keep cannot preserve
+            // them today. That gap (and the related dropping of internal blank
+            // lines and folded-paragraph breaks) is tracked separately. Strip is
+            // the only branch that omits the final break.
             return content + "\n"
         }
     }

Tests/ScannerTests.swift

@@ -333,7 +333,7 @@ private func expectFlowAndBlockScalarKinds(_ tokens: [String]) {
         "mappingKey",
         "scalar value=\"text\" style=plain",
         "mappingValue",
-        "blockScalarHeader style=literal",
+        "blockScalarHeader style=literal chomping=keep",
         "indent width=2",
         "scalar value=\"line\" style=plain",
         "dedent width=0",
@@ -357,7 +357,7 @@ private func expectFlowAndBlockScalarRequiredKinds(_ tokens: [String]) {
         "tag value=!Thing",
         "anchor name=item",
         "alias name=item",
-        "blockScalarHeader style=literal",
+        "blockScalarHeader style=literal chomping=keep",
         "blockScalarHeader style=folded chomping=strip",
     ])
 }