test(coverage): Add comprehensive unit tests to improve coverage

Added 28 new unit tests to increase code coverage from 79.11% to
80.68%, bringing it closer to the 82% target. Total test count
increased from 130 to 158 tests.

New test coverage:
- #[cfg(test)] attribute detection on functions
- File size limit enforcement and error paths
- Directory analysis edge cases (no files, all too large)
- Mixed file/directory analysis workflows
- Output formatting (text/JSON) with both accumulator types
- End-to-end integration workflows
- Multi-file and subdirectory analysis
- FileBackedAccumulator operations and iteration
- Report serialization/deserialization
- Args::parse_max_file_size error handling

Remaining 1.32% gap to 82% consists of code that cannot be easily
unit tested:
- main() function (74 lines, requires integration tests)
- Debug logging statements (require env var configuration)
- Error handling for system failures (temp file I/O errors)
- Progress bar terminal detection logic
- println! macros (executed but not counted by tarpaulin)

All tests pass (158/158) and clippy reports no warnings.

$test

Author: behrangsa