docs: add demos and update documentation for new features

Documentation:
- Update IMPLEMENTATION_PLAN.md with current status (222 tests)
- Update feature comparison table (Schema Evolution, Behavioral Types: ✅)
- Add HttpServer to built-in effects list
- Update OVERVIEW.md with working behavioral types examples

Demo Programs:
- examples/schema_evolution.lux - Version annotations, constraints, runtime ops
- examples/behavioral_types.lux - pure, deterministic, commutative, idempotent, total

Sample Project:
- projects/rest-api/ - Full REST API demo with:
  - Task CRUD endpoints
  - Pattern matching router
  - JSON serialization
  - Effect-tracked request handling

Tests:
- Add behavioral type tests (pure, deterministic, commutative, idempotent, total)
- 227 tests passing

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

projects/rest-api/README.md

@@ -0,0 +1,84 @@
+# REST API Demo
+
+A simple task management REST API demonstrating Lux's HTTP server effect and effect tracking.
+
+## Features Demonstrated
+
+- **HttpServer Effect**: Built-in HTTP server with effect tracking
+- **Pattern Matching**: Route handling via pattern matching
+- **JSON**: Serialization and parsing
+- **ADTs**: `ApiResponse` type with Success/Error variants
+- **Effect Signatures**: All side effects explicitly declared
+
+## Running
+
+```bash
+cargo run -- projects/rest-api/main.lux
+```
+
+## API Endpoints
+
+| Method | Endpoint      | Description       |
+|--------|---------------|-------------------|
+| GET    | /             | API info          |
+| GET    | /tasks        | List all tasks    |
+| GET    | /tasks/:id    | Get task by ID    |
+| POST   | /tasks        | Create new task   |
+| PUT    | /tasks/:id    | Update task       |
+| DELETE | /tasks/:id    | Delete task       |
+
+## Example Usage
+
+```bash
+# Get API info
+curl http://localhost:8080/
+
+# List tasks
+curl http://localhost:8080/tasks
+
+# Get specific task
+curl http://localhost:8080/tasks/1
+
+# Create task
+curl -X POST -d '{"title":"New task","done":false}' http://localhost:8080/tasks
+
+# Update task
+curl -X PUT -d '{"done":true}' http://localhost:8080/tasks/1
+
+# Delete task
+curl -X DELETE http://localhost:8080/tasks/1
+```
+
+## Code Structure
+
+```
+main.lux
+├── Data Types (Task, ApiResponse)
+├── JSON Serialization (taskToJson, tasksToJson)
+├── Route Handlers (handleGetTasks, handleCreateTask, etc.)
+├── Router (route function with pattern matching)
+├── Request Handler (handleRequest)
+├── Server Loop (serveRequests - recursive)
+└── Main Entry Point
+```
+
+## Effect Tracking
+
+All functions declare their effects explicitly:
+
+```lux
+fn handleRequest(req: Request): Unit with {Console, HttpServer} = ...
+fn serveRequests(count: Int, max: Int): Unit with {Console, HttpServer} = ...
+fn main(): Unit with {Console, HttpServer} = ...
+```
+
+This ensures:
+- Side effects are visible in function signatures
+- Testing is easier (swap effects for mocks)
+- Compiler verifies effect usage
+
+## Notes
+
+- Server handles 10 requests then stops (for demo purposes)
+- Uses in-memory "database" (hardcoded tasks)
+- Simplified JSON parsing for demonstration