fix(parsing): accept a tab after the colon value indicator

A tab is separation white space in YAML, so it follows the `:` mapping-value
indicator exactly as a space does. isMappingValueBoundary (flow) and
isBlockMappingValueBoundary (block) accepted only a space, line break, comma,
or closing bracket, so `foo:\tbar` folded into the plain scalar `foo:\tbar`
instead of the mapping `{foo: bar}`.

Accept a tab in both. A `:` with no following white space (`foo:bar`) is still
plain scalar content, and the symmetric counterpart for a tab after a leading
indicator landed in the previous change.

Raises yaml-test-suite parse-level to 300/333 (90.1%) with the should-error
rejection rate unchanged. 433 tests pass.

CHANGELOG.md

@@ -84,6 +84,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- A tab now follows the `:` mapping-value indicator the way a space does, in both block and flow
+  context, since a tab is separation white space in YAML. `foo:\tbar` is the mapping `{foo: bar}`
+  rather than the plain scalar `foo:\tbar`. A `:` with no following white space at all (`foo:bar`)
+  stays plain scalar content. Raises yaml-test-suite parse-level conformance to 300/333 (90.1%), with
+  the should-error rejection rate unchanged.
+
 - A tab now ends a leading indicator the way a space does, since a tab is separation white space in
   YAML (only indentation forbids a tab). A document marker, block entry, or explicit key followed by a
   tab (`---\tscalar`, `-\titem`, `?\tkey`) is recognized as the indicator rather than folded into a

Sources/Parsing/Scanner.Helpers.swift

@@ -53,7 +53,9 @@ extension PureYAML.Parsing.Scanner {
         guard let character else {
             return true
         }
+        // A tab is separation white space, so it follows a `:` value indicator as a space does.
         return character == " "
+            || character == "\t"
             || isLineBreak(character)
             || character == ","
             || character == "]"
@@ -64,7 +66,7 @@ extension PureYAML.Parsing.Scanner {
         guard let character else {
             return true
         }
-        return character == " " || isLineBreak(character)
+        return character == " " || character == "\t" || isLineBreak(character)
     }
 
     func isTokenTerminator(_ character: Character) -> Bool {

Tests/MappingValueTabBoundaryTests.swift

@@ -0,0 +1,34 @@
+@testable import PureYAML
+import Testing
+
+/// A tab is separation white space in YAML, so it follows the `:` mapping-value indicator as a space
+/// does, in both block and flow context. A `:` with no following white space at all stays plain
+/// scalar content (`foo:bar`), unchanged.
+@Suite("Mapping value tab boundary")
+struct MappingValueTabBoundaryTests {
+    @Test("A block mapping value may follow the colon after a tab")
+    func test_blockValueTab() throws {
+        #expect(try PureYAML.parse("foo:\tbar\n") == .mapping(.init([
+            .init(key: "foo", value: .string("bar")),
+        ])))
+    }
+
+    @Test("A block sequence item mapping accepts a tab after the colon")
+    func test_blockSequenceItemTab() throws {
+        #expect(try PureYAML.parse("- foo:\tbar\n") == .sequence([
+            .mapping(.init([.init(key: "foo", value: .string("bar"))])),
+        ]))
+    }
+
+    @Test("A flow mapping value may follow the colon after a tab")
+    func test_flowValueTab() throws {
+        #expect(try PureYAML.parse("{a:\tb}\n") == .mapping(.init([
+            .init(key: "a", value: .string("b")),
+        ])))
+    }
+
+    @Test("A colon with no following white space stays plain scalar content")
+    func test_colonWithoutWhitespaceUnchanged() throws {
+        #expect(try PureYAML.parse("foo:bar\n") == .string("foo:bar"))
+    }
+}