Follow-ups from #375-#384: accepted-diff hand-over format, selection and window edge cases #385

Verification of #375 to #384 (the fixes for #374 and #363) found these. Each one is on main before those PRs too, or is a narrow case the PRs did not claim to cover. Items 1 to 3 come first.

1. Accepted-diff hand-over keeps the buffer's file format (fix(diff): let :ClaudeCodeCloseAllDiffs finish an accepted diff #376). :ClaudeCodeCloseAllDiffs puts an accepted diff's text into the file's buffer. The buffer keeps 'fileformat', 'fileencoding' and 'bomb', so a CRLF proposal in a dos buffer is written as \r\r\n, and a € handed into a latin1 buffer is written with a conversion error. No text is lost: the user sees ^M before :w.
2. Hand-over messages give the wrong reason (fix(diff): let :ClaudeCodeCloseAllDiffs finish an accepted diff #376). A swap file from another Neovim (E325 inside bufload), a failed nvim_buf_set_lines, or a ClaudeCodeDiffClosed handler that accepts a diff during the loop all print "its buffer has unsaved changes or cannot be written". A new file in a missing folder fails :w with E212 and needs a :w ++p hint. A failed hand-over still leaves buflisted/bufhidden changed, and fixeol=false stays after a hand-over.
3. A rejected diff whose client is gone is never cleared. :ClaudeCodeCloseAllDiffs skips it. A full plugin reload also leaves an accepted diff's proposal buffer open but untracked.
4. Selection tracking.
   • A <cmd>ClaudeCodeSend mapping in Select mode sends without a range (visual_commands treats only v, V and CTRL-V as Visual).
   • With 'virtualedit' all, Neovim yanks trailing spaces that the sent text lacks.
   • On Neovim 0.11+, C1 controls and format characters (U+200B) are their own character like TAB, but the plugin joins them with a following combining mark.
   • Blockwise selections are sent as one contiguous span, not the rectangle.
   • PROTOCOL.md's exclusive paragraph does not say that an end on an empty row is character 1.
5. openFile and window choice.
   • A register prefix ("a) looks like Normal mode, so the user's next command (p) acts in the opened file. Documented, not fixable from the mode alone.
   • i<C-o>v takes the normal path, and Insert mode can resume in the opened file.
   • Previewing the buffer a diff window shows opens a closed diff fold in the user's window.
   • With only the preview window and a terminal, openFile and openDiff split the 12-line preview window.
   • preview: true with makeFrontmost: false, when the main window already shows the file, leaves no selection on.
   • With no editor window, makeFrontmost: false still moves focus to the split it creates.
   • The legacy diff.open_diff fallback (wincmd t/l) has no preview-window check. Nothing in the plugin calls it.
   • Doc gaps: lua/claudecode/README.md Diffs line omits the new-tab and fallback-split paths, and tools/README.md names only file-explorer sidebars (the code also skips outline sidebars).
6. Outbound and inbound UTF-8. A buffer name with invalid bytes reaches Claude as U+FFFD, so Claude cannot reopen that exact name. The inbound frame check (server/utils.lua is_valid_utf8) is looser than the strict outbound utils.valid_utf8 (accepts overlong forms, surrogates, F5-FF).
7. verify-claudecode-nvim skill. Under a default parent dir with a default ACL, a new $OUT is briefly 770 before chmod 700 (empty at that time). SKILL.md still says the proof takes about 30 s (now 45 s).
8. CI runs busted in file order only. --shuffle with a printed seed would catch new order dependence.

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