test: improve test coverage to 90%+ (#5)

* test: improve test coverage to 90%+

Expand test suite from 17 tests (22% coverage) to 269 tests (94% coverage).

New test files:
- test_link_traverser.py (~50 tests)
- test_report_generator.py (22 tests)
- test_visual_comparison.py (~57 tests)

Rewritten test files with comprehensive coverage:
- test_cli.py (38 tests)
- test_diff_engine.py (41 tests)
- test_fetcher.py (16 tests)
- test_wayback_cleaner.py (26 tests)

Bug fixes found during testing:
- diff_engine.py: fix compare_structures TypeError on Python 3.14
  (SequenceMatcher rejects unhashable dict elements)
- wayback_cleaner.py: fix remove_wayback_header fallback path
  skipping content past the meta tag boundary

* fix(ci): add Pillow to requirements.txt

visual_comparison.py imports PIL which requires Pillow. It was in
setup.py but not in requirements.txt used by CI.

* fix: add future annotations import for lazy type evaluation

Resolves NameError when selenium is not installed by making
type annotations lazily evaluated via __future__.annotations.

* fix: add selenium to dev dependencies for visual comparison tests

Tests import selenium directly (TimeoutException, webdriver mock
targets), so it must be present in the test environment.

* fix: add webdriver-manager and numpy to dev dependencies

Tests mock ChromeDriverManager/GeckoDriverManager and import numpy
directly, so both must be present in the test environment.

Author: GeiserX

requirements-dev.txt

@@ -1,3 +1,6 @@
 pytest>=7.4.0
 pytest-cov>=4.1.0
 pytest-mock>=3.11.0
+selenium>=4.10.0
+webdriver-manager>=4.0.0
+numpy>=1.24.0

requirements.txt

@@ -1,3 +1,4 @@
 requests>=2.31.0
 beautifulsoup4>=4.12.0
 lxml>=4.9.0
+Pillow>=10.0.0