Preserve view regions across undo

[Shlummie]

Summary

• snapshot a view's marked regions when an undoable edit begins and after it is applied
• restore saved regions on undo and applied regions on redo, including glued undo entries
• add a regression test covering regions restored after deleting the buffer and undoing

Test plan

• gofmt -w edit.go undo.go view.go view_test.go
• go test ./... (downloads dependencies, then fails in this Windows environment on existing native/test setup issues: missing oniguruma for rubex, malformed legacy watcher test names under Go 1.26, and existing package watcher expectations)
• CGO_ENABLED=0 go test . -run TestUndoRestoresRegions -count=1 (fails because github.com/limetext/sublime/textmate requires rubex CGO symbols)

Fixes #101