feat: Add projection validation and error recovery

[jwilger]

Context

During the implementation of multi-stream query patterns (#148), it was noted that projections should handle invalid or corrupted data gracefully. Currently, projections may panic or produce incorrect results if they encounter unexpected data.

Requirements

1. Validation: Projections should validate incoming events
2. Error Recovery: Continue processing valid events even if some are invalid
3. Diagnostics: Report which events failed and why
4. Idempotency: Support re-running projections safely

Proposed Solution

pub enum ProjectionError {
    InvalidEvent { event_id: EventId, reason: String },
    StateCorruption { details: String },
    // ...
}

pub struct ProjectionResult<T> {
    pub state: T,
    pub errors: Vec<ProjectionError>,
    pub processed_count: usize,
}

impl<T> ProjectionBuilder<T> {
    pub fn with_error_handler<F>(self, handler: F) -> Self 
    where F: Fn(ProjectionError) + Send + Sync + 'static {
        // ...
    }
}

Benefits

• Resilient to data corruption
• Better debugging and monitoring
• Can process partial data sets
• Production-ready error handling

Tasks

• Design error types for projections
• Add validation to projection functions
• Implement error collection and reporting
• Add recovery strategies (skip, retry, compensate)
• Create tests for error scenarios
• Document error handling patterns

Labels

• enhancement
• error-handling
• production-readiness