Conversion script now loads corpus XML directly from the upstream
repository URL instead of requiring local files. Parquets regenerated
from authoritative source to pick up any minor differences.
Replace the old tokens-as-segments / integer-entity-id format with the
standard text (plain str) + entity ([[start, end], ...] char offsets) format
used by all other datasets in the project.
Spans are derived from the causality-identification tagged text so the
conversion no longer requires the original XML source files.
added causal-candidate-extraction and the conversion script
