Default to UTF-8 encoding

Baseline: 7338078f8ddd924b8cddee35f512f2286072136d, FileTextLines.cpp::CheckUnicodeType, SetMainPage.cpp::UseUTF8 and IDD_SETMAINPAGE.

Merge Editor General now includes the source Default to UTF-8 encoding checkbox, default false, with shared Settings Apply/Cancel. It uses the same source classifier already ported for commit-message completion scanning. Seven-bit text normally loads as ASCII (Windows-1252); enabling the preference selects UTF-8. BOM-marked Unicode and valid non-ASCII UTF-8 retain their formats. The source's short-file quirk is retained: empty/one/two-byte files classify as ASCII even when the preference is enabled. Strict decoding rejects malformed Unicode and NUL/binary data rather than introducing replacement characters.

ASCII/legacy decoding uses the explicit existing Windows-1252 adapter. Legacy bytes such as é can now open in native two-file and three-file viewers and save in their detected encoding without changing input bytes or adding a BOM. Source Windows ACP depends on the Windows locale; other legacy code pages remain open work. Generic comparison decoding APIs keep their previous defaults. Conflict and standalone three-way APIs opt into Merge classification only when supplied with the new parameter.

The preference is captured when loading/reloading a document. Applying settings does not silently reinterpret an existing draft or its output encoding. Explicit Encoding choices still take priority for that open document and are retained through Save. A reload detects the saved bytes again, as upstream does. Commit completion scanning reads the same canonical preference, with the prior Merge.UseUTF8 key accepted only when TurtleGitMerge.UseUTF8 is absent.

Focused Core checks cover ASCII/short files, source default precedence, Unicode BOMs, valid non-ASCII UTF-8, legacy bytes, malformed/binary rejection, document metadata/byte retention and exact legacy saves. Native checks cover the actual checkbox Apply/Cancel, load/reload timing, two/three-pane legacy decoding and standalone CP1252 output with inputs retained. Full physical settings/format interaction, Windows locale equivalence, malformed-file compatibility, all load callers, signed/minimum-OS and complete application acceptance remain open. This page is still a partial source replacement.

Evidence

The focused Core check passed four tests with zero failures after an initial build-only local-name correction. The unsigned Debug build and all fifteen Merge receiver families passed; one fixture compilation linked the existing app objects and one process ran the checks. See the batch record. Owned windows/helpers closed. No full hosted/signed acceptance is claimed.