Implement atomic multi-stream event append across all event store implementations

[qodo-free-for-open-source-projects]

Description

• Enable appending events to multiple streams in a single atomic operation
• Provide store-specific implementations leveraging native atomic capabilities where available
• Maintain backward compatibility with existing single-stream append operations
• Support both application-layer and low-level event append patterns

Deliverables

• Core Interface: Add NewStreamAppend type and multi-stream AppendEvents overload to IEventWriter with sequential fail-fast default
• KurrentDB Support: Implement native atomic multi-stream append using MultiStreamAppendAsync (requires KurrentDB 25.1+)
• SQL Implementations: Add atomic multi-stream append via single database transaction for PostgreSQL, SQL Server, and SQLite
• Decorator Support: Extend TracedEventWriter, TracedEventStore, and TieredEventStore to handle multi-stream operations
• Application Extensions: Provide Store extension methods for ProposedAppend[] and raw event collections
• Test Coverage: Add base test class with three test cases inherited by all store implementations
• Documentation: Update event store documentation with atomicity guarantees per implementation

[linear]

EVE-221