fix: avoid re-inserting previous filter header on sync resume

[xdustinface]

On restart, start_download() unconditionally set checkpoint_start_height which caused process_cfheaders() to store previous_filter_header at an offset that already contained data, triggering a debug_assert panic in debug builds.

Only set checkpoint_start_height when filter storage is empty not when resuming from existing stored state.

Summary by CodeRabbit

• Bug Fixes
  • Improved filter header checkpoint synchronization to prevent duplicate header re-insertion when resuming interrupted syncs, enhancing sync reliability.

[coderabbitai]

📝 Walkthrough

Walkthrough

The filter headers manager's start-download path now only sets checkpoint_start_height when both conditions are met: no existing filter headers exist (filter_headers_tip == 0) and start_height is greater than 0. This change prevents potential re-insertion of the previous filter header upon checkpoint sync resume.

Changes

Cohort / File(s)	Summary
Filter Headers Manager dash-spv/src/sync/filter_headers/manager.rs	Modified start-download checkpoint logic to only set checkpoint_start_height when filter_headers_tip == 0 and start_height > 0, preventing unintended re-insertion on sync resume.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Poem

🐰 A filter header stands its ground,
No re-insertion shall be found,
When checkpoints fresh begin their quest,
We guard the state, we know what's best,
One small condition makes it right,
Preventing bugs in blockchain's light! ✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The PR title clearly and concisely describes the main fix: preventing re-insertion of a previous filter header when syncing resumes, which directly matches the changeset's core objective.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/filter-sync-prev-header

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