fix: validate ReceiptBuilder ngram and threshold

[HarshRajSinghania]

Summary

Validate ReceiptBuilder construction parameters so degenerate citation settings cannot silently produce a perfect grounding score.

Motivation

Fixes #20. ngram=0 produces an empty gram set that matches every claim (grounding_score 1.0). threshold was also accepted outside [0, 1].

Implementation

• ReceiptBuilder.__init__ now raises ValueError when ngram < 1 (including 0 and negatives) or when threshold is outside [0, 1].
• Boolean values are rejected for both parameters because bool is a subclass of int.
• Default ngram=3 / threshold=0.5 behavior is unchanged.

Testing

Ran locally:

PYTHONPATH=src pytest tests/ -q

Result: 96 passed.

Added tests in tests/test_builder.py for ngram=0, negative ngram, threshold=1.5, threshold=-0.1, and boundary values 0.0/1.0.