Rewrite the 'netlab restart' functionality

[ipspace]

The 'netlab restart' functionality was mostly broken and did not work with the 'more recent' additions like non-default snapshot files or explicit lab instances.

This commit completely rewrites the top-level 'netlab restart' workflow and hopefully makes the code and documentation match.

Closes #3359 and replaces #3354

[ipspace]

@sjhloco -- in the end, I rewrote the whole stuff. The old code was obsolete and way too trivial (I remember someone asking for it and me hacking it together in a minute or two). Would love to hear whether it works for you.

@DanPartelly -- do you want to look at this?

[Copilot]

Pull request overview

This PR rewrites the netlab restart command flow to reliably restart a lab by loading the snapshot (optionally from a selected instance) to discover the original topology file, then running netlab down followed by netlab up. It also adds a platform integration test and updates the user documentation to reflect the new behavior.

Changes:

• Reimplemented netsim/cli/restart.py to load the snapshot, stop the lab, and restart using the original topology path stored in the snapshot; added --instance support.
• Added a platform integration CLI test that exercises netlab restart.
• Updated restart documentation and help/usage text to match the revised workflow and options.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
netsim/cli/restart.py	New restart workflow: parse args, load snapshot to find original topology, call down.run() then up.run() with reconstructed args.
tests/platform-integration/cli/11-restart.yml	Adds an integration scenario that brings up a lab and runs netlab restart, validating OSPF adjacency afterward.
docs/netlab/restart.md	Updates command description and usage/options to document snapshot/instance-aware restart behavior.

[sjhloco]

@sjhloco -- in the end, I rewrote the whole stuff. The old code was obsolete and way too trivial (I remember someone asking for it and me hacking it together in a minute or two). Would love to hear whether it works for you.

Yes that now works, thanks a lot for fixing 👍

[ipspace]

Yes that now works, thanks a lot for fixing 👍

Thanks for the confirmation, merging...