fix: prevent stale session artifacts from restoring cleared drawings

[devmobasa]

Resolves #232

Summary

Fixes session persistence recovery paths that could restore stale drawings after an intentional clear, or delete/shadow the only recoverable backup/recovery artifact during contentless saves.

Changes include:

• Add clear markers to suppress stale primary, backup, and recovery artifacts.
• Preserve backup-only and recovery-only board data unless the user explicitly cleared loaded board data.
• Track whether board data was loaded/saved in runtime session state before treating contentless saves as clear boundaries.
• Add recoverable sidecar markers for protected backup/recovery artifacts.
• Expand session clear cleanup to remove all related marker/recovery variants.
• Avoid overwriting a valid backup when probing a corrupt primary suppressed by a clear marker.