fix: preserve Java normalization semantics for line terminators - #507

Normalizing a vanity number containing a line terminator currently converts its letters to keypad digits even though Java drops those letters. For example, Normalize("1800FLOWERS\n") returns 18003569377 in C# and 1800 in Java. This is a P2 parity defect with high confidence, affecting callers of normalization with copied or multiline input.

The string and StringBuilder guards now reject the five line terminators excluded by Java's non-DOTALL VALID_ALPHA_PHONE_PATTERN: LF, CR, NEL, line separator and paragraph separator. Ordinary spaces, tabs, vertical tabs and form feeds continue to allow vanity conversion. The additional scan runs only after finding three ASCII letters; no regex or public API is added.

Java reference: PhoneNumberUtil.java at 33be5664343676b27761213d8f5500d682568ea7, specifically VALID_ALPHA_PHONE_PATTERN and normalize(StringBuilder) at lines 804–811. The existing normalization tests are in the corresponding PhoneNumberUtilTest.java.

Validation:

• Freshly compiled the Java baseline and executed 22 equivalent cases: six terminator forms at three positions, plus four whitespace controls. All passed.
• Added a C# regression to the corresponding synthetic-metadata test class; observed it fail before the fix with expected 1800, actual 18003569377, and pass afterward.
• dotnet test csharp/PhoneNumbers.slnx: 1,008 tests passed (453 library + 51 extensions tests on each of net8.0 and net10.0).
• dotnet build csharp --no-restore: all three library TFMs built, zero warnings/errors, including trim/AOT analyzers.
• Executed the same 22 cases against the modern DLL and the hash-verified netstandard2.0 DLL using an external harness; all passed.
• Before/after PhoneNumberWorkflowBenchmark.ParseOnly over 1,000 numbers: 358.6 / 335.5 microseconds, 348.12 KB allocated in both. Five measured iterations, one launch each; broad timing intervals overlap, so this is a limited regression check, not evidence of a speedup or of vanity-specific throughput.
• Release package validation and clean-rebuild reproducibility checked with CI=true and VersionPrefix=9.0.42; all three packaged DLL hashes match across both builds.
• Diff whitespace checked with core.whitespace=cr-at-eol, preserving the files' existing line endings.

Review baselines: C# main dcaee5a42ab993d26a4bac47ef1cac4f1150ade2; Java master 33be5664343676b27761213d8f5500d682568ea7. The C# metadata is v9.0.42 and Java's checked-in metadata is v9.0.41; these comparisons do not consult metadata. This review does not establish whole-library parity. No Java source or generated metadata changed.