fix: start context folds closed and keep open ones open

[undont]

Summary

• a finite context value builds with folds closed. previously they were created open, only collapsing explicitly with zM, and every file switch reopened them
• a fold you open stays open through a context size change, a layout toggle, a hunk revert and a re-read/re-render of the file, and comes back open when you return to its diff later within the same session
• opened folds are tracked as the old/new line ranges of their unchanged runs, matched on whichever side's text is unchanged, so a re-read that adds, drops or moves hunks keeps the stretch you opened; the renderers' gap index got removed accordingly
• when differ swaps the diff without you navigating (staging the last unstaged hunk, df on a staged diff, the watcher moving to the other pair), set_source gets keep_folds and carries the folds on screen
• context accepts "full", now the default (math.huge still works); an invalid value warns and falls back to it
• :Differ context and :Differ layout work from the file panel, where they reported "no diff view here"

closes #58

Test plan

• make check clean