test: add architecture tests with NetArchTest.eNhancedEdition

33 architecture tests covering:
- Layer isolation: library must not depend on CLI or Gnu.Getopt
- IFormatter: at least 2 implementations, all in System.Net namespace
- ActionOutput: abstract base, all subtypes sealed, override WriteTo,
  no public constructors (closed hierarchy), at least 8 subtypes
- IPNetwork2: sealed, implements ISerializable
- ICidrGuess: all implementations sealed, at least 3
- Static classes: Program and ActionComputer must be static
- Naming: interfaces start with 'I', enums are public
- Namespaces: all types in System.Net
- DTOs: ContainInfo, OverlapInfo, UsageOptionGroup, UsageOption sealed
- JsonFormatter: no System.Console dependency
- Extension classes: must be static
- Collections: implement IEnumerable<T>
- ProgramContext: public parameterless constructor
- Tests: public, no Gnu.Getopt dependency

Also adds 18 CLI coverage tests (line coverage 89% → 94%).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: lduchosal