feat(buffer-file): legible save prompts for save-some-buffers and disk-changed saves

Replace the cryptic single-key save prompts with read-multiple-choice menus whose options are labeled on screen instead of recalled as keys.

save-some-buffers (C-x s and save-on-exit) is overridden with a reimplementation that reuses the stock candidate selection and abbrev-save tail but swaps map-y-or-n-p's terse key list for a labeled menu, and adds a clean-whitespace-and-save action.

C-x C-s gains a wrapper that fast-paths normal saves to save-buffer; only when the buffer has unsaved edits and the file changed on disk does it show a labeled menu (save / diff / clean / revert / cancel) instead of the bare "Save anyway?" yes/no.

Both menus share a diff toggle, and whitespace-only diffs route to a plain unified diff with trailing whitespace highlighted, since difftastic normalizes trailing whitespace and renders such changes blank.

The interactive wrappers stay thin over pure, tested helpers: the save-loop planner, the key-to-action maps, the whitespace detector, and the renderer choice.

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