Perf: remove Regex in zip entry lookup and move I/O outside lock in geocoder

[Copilot]

Two inefficiencies in PhoneNumberOfflineGeocoder — one creating unnecessary Regex objects per zip entry comparison, another holding a lock across slow file I/O.

Changes

• Replace Regex.Replace with string ops in GetManifestZipFileStream: the static Regex.Replace call internally constructs a Regex object on every zip entry during each geocoding lookup. Replaced with string.Replace(char, char) — no allocation, no compilation overhead.

  // Before
  archive.Entries.First(p => Regex.Replace(p.FullName, "[\\\\/]", ".") == fileName);
  
  // After
  archive.Entries.First(p => p.FullName.Replace('\\', '.').Replace('/', '.') == fileName);

• Move file I/O outside the lock in GetPhonePrefixDescriptions: previously the availablePhonePrefixMaps lock was held for the full duration of zip decompression + parsing, serializing all threads behind a single load. Changed to double-check locking: cache lookup under lock → load without lock → store under lock. Concurrent requests for different locales no longer block each other.

• Removed unused using System.Text.RegularExpressions following the Regex removal.

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