feat: add batch export, crash recovery and progress resumption for large projects

[mohammedrashithkp]

Summary

Rewrites the core export pipeline to handle large Fusion 360 projects
reliably. Builds on the document close fix originally attempted by @m79lol.

Problems Fixed

• F360 crashes on large projects due to memory buildup from sequential
  document opens with no cooldown
• On crash, entire export restarts from file 1 with no way to resume
• No progress feedback — UI appears frozen during scan and export
• Double traversal: folder tree scanned once to count, again to export
• Documents not always closed on export error (partial fix by @m79lol)
• Magic index arithmetic (file_types.count - 2) breaks silently if
  format order changes

Changes

Performance

• Single-pass file collection replaces count-then-export double traversal
• Pre-export disk check skips already-exported files without opening docs

Stability

• Configurable batch cooldown pauses between every 5 files to let F360
  release memory (BATCH_SIZE=5, BATCH_COOLDOWN=2.0s)
• Settle delay between document close and next open (OPEN_SETTLE_TIME=0.3s)
• document.close() moved to finally block — always runs even on error
• Named EXPORT_FORMATS dict replaces magic index arithmetic

Crash Recovery

• Progress saved to archiver_progress.json after every batch
• Rerunning the plugin resumes from last completed file automatically
• Progress file deleted on successful full run

UX

• Progress dialog shows immediately with per-file status updates
• Cancellation supported mid-run with progress saved
• Final summary shows exported vs skipped file counts

Tuning

Constants at the top of the file can be adjusted per project size:

Constant	Default	Purpose
BATCH_SIZE	5	Files per batch before cooldown
BATCH_COOLDOWN	2.0s	Pause between batches
OPEN_SETTLE_TIME	0.3s	Delay after closing a document