update: each changed file lives on one shared row, and stage hunks the index holds part of

[undont]

Summary

• a file changed in both the index and the worktree lists once, under a new Partial section, instead of once per side. every row diffs HEAD to the worktree with the lines the index already holds shaded; sections come from git's status letters: Conflicts, Staged, Partial, Unstaged, Untracked
• staged state is tracked per line, so s and u act on a hunk the index holds only part of, and the winbar tallies staged and partial hunks
• staging no longer patches the index: each op works out the content the index should hold and writes it whole, so a hunk can't land on the wrong side of a context line
• staged content the diff can't draw (staged then put back, staged then edited again, a staged mode change undone on disk) gets a ! in the gutter and a winbar note. dw opens what changed since staging, where it shows as hunks of its own and s / u / X work on it, including a mode change, a binary file and a staged add whose file was deleted
• u on a ! hunk takes back only the lines it rewrote, and names the whole staged block when it can't
• ds previews the commit: the panel lists only the staged files, each diffed HEAD to the index, and u takes changes back out. X there refuses rather than discard unstaged work the preview doesn't show
• X on a hunk discards it from the index and the file together, rather than asking you to unstage it first
• X reverts a mode change or a binary file from the diff, and brings a staged edit whose file was deleted back as staged, in the diff and the panel alike
• u and U ask before dropping staged content that neither HEAD nor the file holds, in the diff and the panel alike
• S / U take the whole file to the worktree's or HEAD's side, mode included; a rename's move stays with the panel row
• the s / u walk goes down the list once and ends by saying what is left. s stops on a file whose remaining change only dw shows, until dw finishes it or ]f leaves it
• editing from a staged diff no longer unstages the whole file first
• a change to a file's final newline is a hunk on the last line, with a ¬ in the gutter on whichever side ends without one
• some other small fixes: paths with glob characters or a leading 1: no longer stage the wrong content, symlinks diffing as their target, discarding a staged rename or add whose file was deleted, "stage failed" on a staged rename or deletion from the panel, a file swapped for a symlink refusing every staging key, and panel X silently doing nothing on a submodule

Test plan

• make check clean
• driven headless against a scratch repo covering MM, RM, RD, AD, MD, kept deletions, symlinks, glob and colon paths, a merge conflict and all four final-newline cases, with both review walks run end to end
• s / S / u / U / X walked headless across the whole diff, whole-file rows, the local view, the INDEX view and the commit preview; each finding fixed with a spec
• ~20,000 fuzzed ops (s / S / u / U / X, dw and ds) across 8 seeds: no lua errors, no write to the wrong path or the worktree, no index line from outside HEAD, the index, the worktree or the open view's snapshot