Add streaming WAL replay with scheduler yields

[bravo1goingdark]

Motivation

• Reduce long, blocking pauses during WAL-based recovery by processing records incrementally and yielding to the scheduler.
• Provide a streaming API to consume WAL records without materializing the full Vec, enabling lower memory usage and backpressure-friendly recovery.
• Preserve the existing iterate_from behavior for tests and callers by implementing it on top of the new streaming API.

Description

• Add a new WriteAheadLog::iterate_from_stream method that takes a callback (WalRecord) -> Future<Output=Result<(), E>> and streams records from the WAL starting at a given id, mapping WAL I/O errors into the callback error type via E: From<WalError> and allowing incremental processing.
• Route the existing iterate_from implementation through iterate_from_stream so the old vector-based API remains available for tests and callers.
• Update Broker::replay_from_wal to use iterate_from_stream and process records one-by-one, calling tokio::task::yield_now().await every yield_every records (set to 1024) to yield to the scheduler and avoid long monopolization of the runtime.
• Remove an unused WalRecord import from core::lib.rs as the broker now decodes records inside the streaming callback.

Testing

• No automated tests were executed as part of this change.
• Existing WAL unit tests remain in the codebase and the public iterate_from behavior is preserved because it delegates to the new streaming API.

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