add unit test coverage for EmptyReplayLogger and ReplayLogGenerator

[aglinxinyuan]

Background

Two modules in engine/architecture/logreplay currently lack a dedicated unit-spec:

Source class	Purpose
EmptyReplayLogger	Null-object ReplayLogger whose logCurrentStepWithMessage / markAsReplayDestination are no-ops and drainCurrentLogRecords returns an empty array
ReplayLogGenerator	Pure decoder: walks a SequentialRecordStorage[ReplayLogRecord] and partitions the records into a (steps, messages) queue pair, stopping at the requested ReplayDestination

Both sit on the fault-tolerance / replay hot path. The ReplayLogger / OrderEnforcer traits and ReplayOrderEnforcer are out of scope here (heavier dependencies — covered separately).

Behavior to pin

EmptyReplayLogger

Surface	Contract
logCurrentStepWithMessage	no-op (returns Unit, no exception)
markAsReplayDestination	no-op
drainCurrentLogRecords	returns an empty Array[ReplayLogRecord] regardless of step
typing	is a ReplayLogger (compile-time enforced)

ReplayLogGenerator.generate

Surface	Contract
empty storage (getStorage(None))	returns (empty queue, empty queue)
storage containing only ProcessingStep records	enqueues all into steps; messages queue is empty
storage containing only MessageContent records	enqueues all into messages; steps queue is empty
storage containing both kinds, interleaved	partitions correctly by type, preserving per-type insertion order
ReplayDestination(id) matching replayTo	short-circuits early — records after that point are NOT enqueued
ReplayDestination(id) NOT matching replayTo	is silently skipped (does NOT end iteration)
unknown record type	throws RuntimeException

Scope

• New spec files (one per source class per the spec-filename convention):
  • EmptyReplayLoggerSpec.scala
  • ReplayLogGeneratorSpec.scala
• No production-code changes.
• The ReplayLogGenerator spec uses an in-memory SequentialRecordStorage (the VFSRecordStorage backed by a temp dir; same pattern as the existing checkpoint specs) and the production AmberRuntime.serde for round-trip.