Create patch file and General context setting

Source baseline: 7338078f8ddd924b8cddee35f512f2286072136d. MainFrm.cpp::OnEditCreateunifieddifffile, AppUtils.cpp::CreateUnifiedDiff, SetMainPage.cpp and IDD_SETMAINPAGE define this workflow.

The two-file viewer provides Create patch file in its toolbar and the native File menu while that comparison owns the key window. The source icon is retained. A native Save panel proposes a .diff file; Cancel writes nothing. Single-file views, busy/closing viewers and attached sheets disable the command. Ordinary three-pane merges do not offer it. Closing or retiring a viewer fences late save replies and cancels owned Git work.

Merge Editor settings now separate source Misc and Diffing groups from additional comparison/view options. Context lines for patches accepts the source range -1 through 1000. -1 omits --unified so Git uses its default; zero omits surrounding lines. Apply persists the value; Cancel restores it. Invalid values block the complete settings transaction. General settings accept isolated preference/profile locations for native verification.

As upstream does, generation uses Git diff --no-index, accepting exits 0 and 1. Working inputs are read from saved files at export time; unsaved editor drafts and display whitespace/case/regex filters do not alter the patch. Historical inputs retain their captured revision bytes. Private temporary inputs preserve regular/executable/symlink modes. Header names retain comparison paths (absolute standalone paths use their file names) and never expose temporary locations. Git C-style quoting preserves spaces, tabs, quotes and Unicode; hunk bytes are never decoded, so legacy text and CRLF remain intact. Missing sides use /dev/null. Output is written atomically with the save-panel lease held.

The source command does not request --binary; binary differences produce Git's diagnostic rather than an applicable binary payload. This behavior is retained.

The combined one-pane workflow/default is now implemented; see one-pane parity. Default to UTF-8 now follows the source classifier; see default encoding parity. Windows Ribbon preferences require a native toolbar adaptation. The Colors page now has native per-channel Automatic/More colors controls, independent palette drafts and Restore Default. See Colors parity for panel lifecycle and remaining theme/physical acceptance. Exact physical page/menu layout, save-panel overwrite/grant acceptance and signed Store sandbox acceptance remain open. Focused checks do not certify the full family.