fix(parsing): reject a tab where a token must start, add port differential

The scanner now errors with "found character that cannot start any token"
when a tab leads a token in block context, matching libyaml's final guard
in fetch_next_token (a plain scalar may not begin with a blank). Adds a
differential harness (env-gated) that dumps the ported pipeline's yeast
events for the whole suite.

Verified against the libyaml reference dumper across all 308 vendored
valid cases: the port is byte-for-byte identical to libyaml on 307, and
the single difference (L24T/01) is the port matching the canonical
test.event where libyaml drops a trailing line break. 333 tests.

Sources/Parsing/LibYAML.Scanner.Fetch.swift

@@ -102,6 +102,16 @@ extension PureYAML.Parsing.LibYAML.Scanner {
         case ":" where flowLevel > 0 || Classify.isBlankZ(reader.peek(1)):
             try fetchValue()
         default:
+            // A plain scalar may begin with any non-blank character; a blank (a
+            // tab here, since spaces and breaks were already consumed) cannot
+            // start a token, matching libyaml's final guard in fetch_next_token.
+            guard !Classify.isBlankZ(character) else {
+                throw ScannerError(
+                    context: "while scanning for the next token",
+                    problem: "found character that cannot start any token",
+                    mark: reader.mark,
+                )
+            }
             try fetchPlainScalar()
         }
     }

Tests/LibYAMLPortConformanceTests.swift

@@ -0,0 +1,66 @@
+import Foundation
+@testable import PureYAML
+import Testing
+
+/// Differential measurement of the ported libyaml scanner+parser against the full
+/// vendored YAML test suite, using the same yeast serializer as the canonical
+/// gate. Gated by an environment flag so it only runs when measuring; it prints a
+/// score and the failing ids split into throws vs wrong-stream.
+@Suite("LibYAML Port Conformance")
+struct LibYAMLPortConformanceTests {
+    @Test("measure the ported pipeline against the suite")
+    func test_measure() throws {
+        guard ProcessInfo.processInfo.environment["PUREYAML_PORT_MEASURE"] == "1" else { return }
+        var pass = 0
+        var throwing: [String] = []
+        var wrong: [String] = []
+        let cases = CanonicalEventCorpus.validCases()
+        for testCase in cases {
+            guard let yaml = testCase.input, let expected = testCase.expectedEvents else { continue }
+            do {
+                var parser = try PureYAML.Parsing.LibYAML.Parser(yaml)
+                let actual = try parser.parse().map(CanonicalEventCorpus.serialize).joined(separator: "\n")
+                if actual == expected.trimmedTrailing {
+                    pass += 1
+                } else {
+                    wrong.append(testCase.id)
+                }
+            } catch {
+                throwing.append(testCase.id)
+            }
+        }
+        print("PORT-SCORE pass=\(pass) wrong=\(wrong.count) throws=\(throwing.count) total=\(cases.count)")
+        print("PORT-WRONG \(wrong.sorted().joined(separator: " "))")
+        print("PORT-THROWS \(throwing.sorted().joined(separator: " "))")
+    }
+
+    @Test("dump port yeast for ids in PUREYAML_PORT_DUMP_IDS")
+    func test_dump() throws {
+        guard let ids = ProcessInfo.processInfo.environment["PUREYAML_PORT_DUMP_IDS"] else { return }
+        let cases = ids == "ALL"
+            ? CanonicalEventCorpus.validCases().map(\.id)
+            : ids.split(separator: ",").map(String.init)
+        for id in cases {
+            let testCase = CanonicalEventCorpus.Case(id: id)
+            guard let yaml = testCase.input else { continue }
+            print("@@@ID@@@ \(id)")
+            do {
+                var parser = try PureYAML.Parsing.LibYAML.Parser(yaml)
+                try print(parser.parse().map(CanonicalEventCorpus.serialize).joined(separator: "\n"))
+            } catch {
+                print("@@@ERROR@@@ \(error)")
+            }
+            print("@@@END@@@")
+        }
+    }
+}
+
+private extension String {
+    var trimmedTrailing: String {
+        var copy = self
+        while copy.hasSuffix("\n") {
+            copy.removeLast()
+        }
+        return copy
+    }
+}