Use generics to deduplicate internal patterns

[klowdo]

Description

Use Go generics to deduplicate four near-identical internal patterns while keeping all public API signatures unchanged. Also adds generic registration alternatives for commands and event data.

Affected Components

• Command registry
• Event data registry
• Aggregate registry
• Snapshot registry
• Middleware chaining
• Context value extraction
• Repo unwrapping (cache, version, tracing)

Related Issues

None

Solution and Design

• A generic typeRegistry[K, F] replaces four copy-pasted mutex/map/panic registry implementations in command.go, event.go, aggregate.go, and snapshot.go.
• A generic applyMiddleware[H, M] deduplicates UseCommandHandlerMiddleware and UseEventHandlerMiddleware.
• A generic fromContext[T] simplifies context value extraction to one-liners.
• A generic IntoRepo[T] eliminates recursive repo unwrapping duplication across repo/cache, repo/version, and tracing.
• New RegisterCommandType[T]() and RegisterEventDataType[T](eventType) provide generic alternatives that eliminate factory closures:

  // Before:
  eh.RegisterCommand(func() eh.Command { return &Create{} })
  eh.RegisterEventData(MyEventType, func() eh.EventData { return &MyEventData{} })
  
  // After:
  eh.RegisterCommandType[Create]()
  eh.RegisterEventDataType[MyEventData](MyEventType)

  The existing factory-based functions remain unchanged for backward compatibility. Aggregates still require the factory approach due to UUID-dependent construction.

All internal changes are backward compatible. No exported type signatures changed.

Steps to test and verify

1. go build ./... — all packages compile
2. go vet ./... — no new issues
3. go test ./... — all existing tests pass, new generic registration tests included

Closes #405