feat: expose neural entity extraction APIs (v5.7.6 - Workshop request)

Addresses Workshop team's request for direct access to neural extraction classes.

**Changes:**

1. **New Exports** (src/index.ts):
   - `NeuralEntityExtractor` - Full extraction orchestrator
   - `SmartExtractor` - Entity type classifier (4-signal ensemble)
   - `SmartRelationshipExtractor` - Relationship type classifier
   - Types: `ExtractedEntity`, `ExtractionResult`, `RelationshipExtractionResult`, etc.

2. **Package.json Subpath Exports**:
   ```typescript
   // Enable direct imports:
   import { NeuralEntityExtractor } from '@soulcraft/brainy/neural/entityExtractor'
   import { SmartExtractor } from '@soulcraft/brainy/neural/SmartExtractor'
   import { SmartRelationshipExtractor } from '@soulcraft/brainy/neural/SmartRelationshipExtractor'
   ```

3. **New brain.extractEntities() Method** (brainy.ts:3254):
   - Alias for `brain.extract()` with clearer naming
   - Documented with examples and architecture details
   - 4-signal ensemble: ExactMatch (40%) + Embedding (35%) + Pattern (20%) + Context (5%)

4. **Comprehensive Documentation** (docs/neural-extraction.md):
   - Complete neural extraction guide (200+ lines)
   - API reference for all extraction classes
   - Performance optimization tips
   - Import preview mode documentation
   - Confidence scoring explanation
   - 42 NounType detection methods
   - Troubleshooting guide
   - Real-world examples

5. **README Updates**:
   - Added "Entity Extraction" section with examples
   - Links to neural extraction guide
   - Import preview mode link

**Features:**
- ⚡ Fast extraction: ~15-20ms per entity
- 🎯 4-signal ensemble architecture
- 📊 Format intelligence (Excel, CSV, PDF, YAML, DOCX, JSON, Markdown)
- 🌍 42 universal noun types + 127 verb types
- 💾 LRU caching built-in
- 🧪 Production-tested in import pipeline

**Usage:**

```typescript
// Simple API (recommended)
const entities = await brain.extractEntities('John Smith founded Acme Corp', {
  types: [NounType.Person, NounType.Organization],
  confidence: 0.7
})

// Advanced API (custom configuration)
import { SmartExtractor } from '@soulcraft/brainy'

const extractor = new SmartExtractor(brain, { minConfidence: 0.8 })
const result = await extractor.extract('CEO', {
  formatContext: { format: 'excel', columnHeader: 'Title' }
})
```

**Backward Compatible:** All existing APIs unchanged. New exports are pure additions.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

package.json

@@ -39,6 +39,18 @@
     "./universal": {
       "import": "./dist/universal/index.js",
       "types": "./dist/universal/index.d.ts"
+    },
+    "./neural/entityExtractor": {
+      "import": "./dist/neural/entityExtractor.js",
+      "types": "./dist/neural/entityExtractor.d.ts"
+    },
+    "./neural/SmartExtractor": {
+      "import": "./dist/neural/SmartExtractor.js",
+      "types": "./dist/neural/SmartExtractor.d.ts"
+    },
+    "./neural/SmartRelationshipExtractor": {
+      "import": "./dist/neural/SmartRelationshipExtractor.js",
+      "types": "./dist/neural/SmartRelationshipExtractor.d.ts"
     }
   },
   "browser": {