fix(sftp): download progress stuck at 0% for files in symlinked directories

[Orinks]

Summary

Download progress shows 0% throughout when downloading files inside symlinked directories on SFTP. Data transfers correctly — only the progress display is broken.

Root cause

Two contributing factors:

1. sftp.stat(remote_path) may return size=0 for files accessed through symlinked directories (server returns no size attribute)
2. For recursive downloads, total size is calculated from f.size values from the READDIR listing — some servers omit size from READDIR attributes, leaving all sizes as 0

Steps to reproduce

1. Connect to an SFTP server that uses symlinked directories (e.g. Bitvise with symlinks)
2. Navigate into a symlinked directory
3. Download a file or directory — transfer completes but progress shows 0% throughout

Expected

Progress bar reflects actual bytes transferred vs total file size.

Notes

• A fix was prototyped (realpath before stat + guard against zeroing total_bytes in callback) but reverted from PR fix(sftp): handle Bitvise strict SFTP compliance for directory navigation #50 as out of scope
• asyncssh reads ARE working correctly — this is purely a size-reporting issue

[Orinks]

Resolved by PR #53 (merged).