Add resumable downloads to get_file

[nickita-khylkouski]

Closes #1617.

Adds resume=True to get_file and get for local path destinations. The generic path opens an existing destination in append mode and seeks the remote file to the same offset. HTTP resumes with an internal range request, restarts safely if the server ignores it, and checks the remote size before accepting a completed local file. There is no public byte-range API.

Tests:

• 8 focused resume/get-file tests passed
• full suite: 1,917 passed, 70 skipped, 2 xfailed
• 52 existing GNU stat -c comparison cases fail on macOS with BSD stat
• Ruff check/format passed on all changed files

[martindurant]

This is a very big PR and hard to review.

I don't mind the idea of resumable downloads (open local file with "ab" and seek in remote file) not byte ranges, although the two conflict with each other. I think the implementation should be significantly simpler than this!