fix(navigation): preserve jumplist before output buffer jumps

[phanen]

Problem: programmatic cursor moves via nvim_win_set_cursor in the
output window (gg, ]], [[, ]u, [u, <CR>, ...) do not update the
previous-position mark, so users cannot return to the prior output
location with `` or <C-o> after a jump.

Solution: introduce mark_jump_position() which records the current
cursor as the ' mark inside the source window via
`noau normal! m'`, and call it before every nvim_win_set_cursor in
navigation.lua and before the gg keymap in output_window.lua.

[sudo-tee]

Thanks for the PR.

This is another great QOL improvement

[phanen]

test seems flaky on master https://github.com/sudo-tee/opencode.nvim/actions/runs/29008327834/job/86085345936

[sudo-tee]

Yes, it's always the same MacOs test that times out.. I will investigate

[sudo-tee]

I pushed a fix, to use a fake timer instead of relying on vim.wait