fix(parsing): skip tabs as mid-line separation white space

skipSeparation consumed only spaces between tokens, so a tab after an
indicator leaked into the following node: an explicit key written `?\tkey`
became the key `\tkey` rather than `key`. A tab is separation white space in
YAML, forbidden only as indentation, which the line-start scan already handles
before this point.

Skip tabs there too. This closes the post-indicator tab leak left open when
the indicator boundary itself was changed, and handles a run of tabs between a
`:` and its value. The test-suite rates are unchanged, since no comparable
case isolates this, but the parser now matches the spec on tab separation.

435 tests pass.

CHANGELOG.md

@@ -84,6 +84,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- Mid-line separation white space between tokens now includes tabs, not only spaces, completing the
+  tab-as-separation handling. An indicator followed by a tab no longer leaks the tab into the next
+  node: `?\tkey` is the key `key`, not `\tkey`, and a run of tabs between a `:` and its value
+  (`a:\t\tb`) is separation. This closes the post-indicator tab leak noted when the indicator boundary
+  itself was fixed. Tabs as indentation are still handled separately by the line-start scan, and the
+  yaml-test-suite rates are unchanged (no comparable case isolates this), but the parser now matches
+  the spec on tab separation.
+
 - 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`)

Sources/Parsing/Scanner.swift

@@ -341,7 +341,9 @@ extension PureYAML.Parsing.Scanner {
     }
 
     func skipSeparation(_ state: inout State) {
-        while state.reader.peek() == " " {
+        // Mid-line separation may be spaces or tabs: a tab is separation white space in YAML and is
+        // forbidden only as indentation, which the line-start indentation scan handles before this.
+        while state.reader.peek() == " " || state.reader.peek() == "\t" {
             state.reader.advance()
         }
     }

Tests/IndicatorTabBoundaryTests.swift

@@ -16,6 +16,20 @@ struct IndicatorTabBoundaryTests {
         #expect(try PureYAML.parse("-\titem\n") == .sequence([.string("item")]))
     }
 
+    @Test("An explicit key indicator followed by a tab starts a mapping key with the tab stripped")
+    func test_explicitKeyTab() throws {
+        #expect(try PureYAML.parse("?\tkey\n: value\n") == .mapping(.init([
+            .init(key: "key", value: .string("value")),
+        ])))
+    }
+
+    @Test("A run of tabs between tokens is separation, not content")
+    func test_multipleTabSeparation() throws {
+        #expect(try PureYAML.parse("a:\t\tb\n") == .mapping(.init([
+            .init(key: "a", value: .string("b")),
+        ])))
+    }
+
     @Test("A space after an indicator is unchanged")
     func test_spaceStillWorks() throws {
         #expect(try PureYAML.parse("--- scalar\n") == .string("scalar"))