feat: stage and unstage a line selection

[undont]

closes #48

Summary

• s / u in visual mode stage and unstage only the selected lines; normal s / u stay hunk-level and finish a partly staged hunk
• split takes whole rows, so a modified line moves with both halves; stacked can take one side alone
• part of a replacement of unequal length stages without a refusal: unselected old lines stay, selected new lines follow them
• refused with a message on files that stage whole, in the dw / ds views, and on hunks holding staged content the diff can't show
• g? notes the visual keys on the hunk row where line staging is available

Test plan

• make check clean