misc fixes before release

- Fix README readability example property names
- Add validation for empty references after tokenisation in ROUGE
- Guard against zero sentence count in readability metric
- Implement LRU cache with max size for semantic embeddings
- Add .score property to LexicalResult for API consistency
- Use defensive list copy in composite validators

readme.md

@@ -110,8 +110,8 @@ from veritext.metrics import Readability
 
 readability = Readability()
 result = readability.score("This is a simple sentence.")
-print(f"Grade level: {result.grade_level:.1f}")
-print(f"Reading ease: {result.reading_ease:.1f}")
+print(f"Grade level: {result.flesch_kincaid_grade:.1f}")
+print(f"Reading ease: {result.flesch_reading_ease:.1f}")
 ```
 
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