Fix FRFR dataset feature naming inconsistency

- Normalize feature names across all 11 experimental conditions
- adaptive condition used 'pos', 'first_letter', 'length'
- Renamed to 'location', 'firstLetter', 'wordLength' for consistency
- All subjects now have consistent feature keys

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: jeremymanning