Add basic stimuli validation example

This example demonstrates core BIDS stimuli structure that validates
successfully with schema-based validation:

- Basic stimuli catalog (stimuli.tsv/json)
- Individual stimulus metadata (stim-*_*.json)
- Proper BIDS field capitalization (Type, Description)
- Clean structure without complex annotations

Purpose: Test case for stimuli BEP validation and development.

Author: neuromechanist

stimuli_basic/README

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+This dataset demonstrates basic stimuli organization according to the BIDS specification.
+
+It includes:
+- stimuli.tsv: Catalog of stimuli with basic metadata
+- stimuli.json: Column definitions for the stimuli catalog  
+- stim-*_*.json: Individual stimulus metadata files
+
+This example validates successfully with the BIDS validator when using a custom schema that supports stimuli validation.
+
+See: https://bids-specification.readthedocs.io/en/latest/modality-agnostic-files/stimuli.html

stimuli_basic/dataset_description.json

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+{
+    "Name": "Basic Stimuli Example",
+    "BIDSVersion": "1.8.0",
+    "License": "CC0",
+    "Authors": ["BIDS Maintainers"],
+    "Description": "Simple example demonstrating basic stimuli structure validation"
+}

stimuli_basic/stimuli/stim-face01_image.json

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+{
+    "Type": "image",
+    "Description": "Neutral face stimulus for emotion recognition task"
+}

stimuli_basic/stimuli/stimuli.json

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+{
+    "stimulus_id": {
+        "Description": "Unique identifier for the stimulus"
+    },
+    "type": {
+        "Description": "Type of stimulus file"
+    },
+    "Description": {
+        "Description": "Free-form description of the stimulus"
+    },
+    "present": {
+        "Description": "Whether the stimulus file is present in the dataset"
+    }
+}

stimuli_basic/stimuli/stimuli.tsv

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+stimulus_id	type	Description	present
+stim-face01	image	A neutral face stimulus	true
+stim-tone01	audio	A 440Hz pure tone	true