model_file_name() produces inconsistent slugs for names with parentheses

[talkasab]

Problem

When a finding model name contains parentheses — e.g., composite metrics (hippocampal occupancy score) — the model_file_name() function produces different slugs depending on context. The index.md generator links to composite_metrics__hippocampal_occupancy_score_.fm.json (double underscore, trailing underscore) but the actual def file is composite_metrics_hippocampal_occupancy_score.fm.json.

This causes broken links in the auto-generated index.

Reproduction

from findingmodel.common import model_file_name
print(model_file_name("composite metrics (hippocampal occupancy score)"))

Expected behavior

model_file_name() should produce a single canonical slug, and the index generator should use the same function to build links.

Found via Copilot review on openimagingdata/findingmodels#48.

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