Add command line switch tests.

[hoche]

Note

Introduces a dedicated command-line parser, refactors main.cpp to use it with validation, and adds extensive unit tests with build integration.

• CLI Parsing:
  • New src/command_line_parser.{h,cpp} implementing option parsing, help output, and ValidateCommandLine (enforces -maxpages constraints and presence of TX sites).
  • src/main.cpp refactored to use CommandLineOptions and ParseCommandLine/ValidateCommandLine; replaces ad-hoc parsing and wires parsed filenames through execution paths.
• Tests:
  • Add tests/command_line_parser_test.cpp with broad coverage of flags, clamping, conflicts, and validation.
• Build System:
  • Include src/command_line_parser.cpp in CMakeLists.txt and test sources; update references in src/CMakeLists.txt.

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