Deduplicate NLTK resource download checks

[shimpeiws]

Summary

ensure_nltk_resources() is called from multiple locations across processor.py and filters.py. Although it is centralized in nltk_resources.py, each call triggers NLTK's internal check, adding unnecessary overhead.

Proposed Approach

• Call ensure_nltk_resources() once at application startup (e.g., in cli.py before analysis begins)
• Alternatively, add a module-level flag in nltk_resources.py so it only executes on the first invocation
• Remove scattered calls from processor.py and filters.py

Affected Files

• src/barscan/analyzer/nltk_resources.py
• src/barscan/analyzer/processor.py
• src/barscan/analyzer/filters.py