add BWS stall detection and step-level debug logging

[macfarla]

• BackwardSyncContext: log when reusing an existing session (vs starting a new one) so stuck sessions are immediately visible
• BackwardSyncContext: emit WARN if a session has made no progress for more than 5 minutes, including session age, time since last progress, and future.isDone() to confirm it is stuck
• BackwardSyncContext: record last-progress timestamp on each block import so the stall timer resets as work proceeds
• BackwardSyncContext: log attempt number on each retry so retry exhaustion is visible at DEBUG
• BackwardSyncAlgorithm: add DEBUG log for each pickNextStep() branch (hash processing, backward fetch, forward sync, known ancestors) so the algorithm's path through each cycle is traceable

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[Copilot]

Pull request overview

Adds stall detection and debug logging to the backward sync subsystem to improve observability of stuck sessions and algorithm step decisions.

Changes:

• Add stall detection warning when a backward sync session makes no progress for 5+ minutes
• Add debug logging for session reuse, retry attempts, and each pickNextStep() branch
• Track last-progress timestamp on block imports to reset the stall timer

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
BackwardSyncContext.java	Stall detection, progress tracking, session reuse logging, retry attempt logging
BackwardSyncAlgorithm.java	Debug logging for each pickNextStep() decision branch

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