Add N400 semantic ERP experiment

[i-vikaas]

Summary

This PR adds a new ERP experiment to elicit the N400 component, based on semantic incongruity during sentence comprehension.

• Experiment name: semantic_n400
• ERP component: N400 — a negative deflection ~400 ms after semantically unexpected words
• Paradigm: Sentence reading task with congruent and incongruent sentence endings
• Use case: Supports research and education on semantic processing using ERP techniques

Dataset

• Includes a simplified stimuli.txt file derived from the Dryad N400 Sentence Stimulus Dataset
• Original dataset: Dryad N400 Sentence Stimulus Dataset
• License: Creative Commons Zero (CC0)

Related Issue

Closes #299