fix(parsing): fold tabs in plain scalar continuation lines

Plain scalar line folding strips white space around continuation lines, but
the scanner stripped only spaces, so tabs leaked in. A tab is not indentation,
so the indentation scan left a continuation line's leading tab in place and it
joined the value (`\t3rd` folded as `\t3rd`, not `3rd`), and a line of only
white space holding a tab (`  \t`) was treated as a non-blank continuation,
folding to a space rather than the line break an empty line gives.

Skip leading white space, tabs included, before a continuation line's content,
and treat a whitespace-only line that holds a tab as blank when counting the
folded line breaks.

Raises yaml-test-suite value-level conformance to 183/194 (94.3%) with the
parse-level and should-error rejection rates unchanged. 441 tests pass.

CHANGELOG.md

@@ -84,6 +84,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- Plain scalar line folding now strips tabs, not only spaces, around continuation lines. A tab is not
+  indentation, so a continuation line's leading tab (`\t3rd`) is folding white space and folds away
+  rather than entering the value, and a line of only white space that holds a tab (`␣␣\t`) is an empty
+  line that folds to a line break, not an extra space. Raises yaml-test-suite value-level conformance
+  to 183/194 (94.3%), with the parse and should-error rates unchanged.
+
 - A tab in block scalar content past the block's indentation is now kept verbatim instead of being
   re-expanded to spaces. A block scalar is indented with spaces only, so a line like `␣\ttext` under a
   one-space indent strips the space and keeps the tab, yielding `\ttext`; the scanner measured the

Sources/Parsing/Scanner.Helpers.swift

@@ -190,8 +190,10 @@ extension PureYAML.Parsing.Scanner {
         var count = 0
         while true {
             var blankProbe = probe
+            // A line of only white space is empty for folding, including one that holds a tab: a tab
+            // is not indentation, but a whitespace-only line is still blank, so consume it here.
             _ = blankProbe.consume { character in
-                character == " "
+                character == " " || character == "\t"
             }
             guard isLineBreak(blankProbe.peek()) else {
                 break
@@ -223,6 +225,12 @@ extension PureYAML.Parsing.Scanner {
     }
 
     func scanPlainScalarContinuationLine(_ state: inout State) -> String {
+        // Leading white space beyond the line's indentation folds away in a plain scalar. A tab is
+        // not indentation, so the indentation scan leaves it here; skip it (and any spaces after it)
+        // so a continuation line like `\t3rd` joins as `3rd`, not `\t3rd`.
+        while let character = state.reader.peek(), character == " " || character == "\t" {
+            state.reader.advance()
+        }
         var value = ""
         while let character = state.reader.peek(), !isLineBreak(character) {
             if character == " ", state.reader.peek(offset: 1) == "#" {

Tests/PlainScalarTabFoldingTests.swift

@@ -0,0 +1,28 @@
+@testable import PureYAML
+import Testing
+
+/// Plain scalar line folding strips white space, including tabs, around continuation lines: a tab is
+/// not indentation, so the leading tab of a continuation line folds away, and a line of only white
+/// space (even one holding a tab) is an empty line that folds to a line break, not a space.
+@Suite("Plain scalar tab folding")
+struct PlainScalarTabFoldingTests {
+    @Test("A leading tab on a continuation line folds away")
+    func test_leadingTabFoldsToSpace() throws {
+        #expect(try PureYAML.parse("1st non-empty\n\n 2nd non-empty \n\t3rd non-empty\n")
+            == .string("1st non-empty\n2nd non-empty 3rd non-empty"))
+    }
+
+    @Test("A whitespace-only line holding a tab is an empty line, folding to a line break")
+    func test_whitespaceOnlyTabLineIsBlank() throws {
+        #expect(try PureYAML.parse("key:\n  value\n  with\n  \t\n  tabs\n") == .mapping(.init([
+            .init(key: "key", value: .string("value with\ntabs")),
+        ])))
+    }
+
+    @Test("A tab after the indentation of a continuation line folds away")
+    func test_tabAfterIndentFolds() throws {
+        #expect(try PureYAML.parse("x:\n - x\n  \tx\n") == .mapping(.init([
+            .init(key: "x", value: .sequence([.string("x x")])),
+        ])))
+    }
+}