Merge command frame continuity

This partial workflow review follows pinned TortoiseGit 7338078f8ddd924b8cddee35f512f2286072136d, src/TortoiseMerge/MainFrm.cpp::OnFileOpen, CWorkingFile::SetFileName and CWorkingFile::TransferDetailsFrom. The native Open dialog corresponds to src/Resources/TortoiseMergeENG.rc / IDD_OPENDLG.

File/Open prepares a fresh native document before transferring the original window. The original command's readonly and Blame policies survive the transfer. The three-file model now carries Base, Theirs, Mine and Merged reflected names independently from its working URLs, matching the frame metadata that SetFileName leaves intact. Two-file replacements retain their corresponding Base/Mine metadata in the current pane orientation; Switch Sides updates that orientation before the next Open. Returning to three files preserves those names and restores the Theirs/Merged names not displayed by the two-file editor. The new document does not inherit the old descriptive labels, explicit output, required-save obligation or initial line. It uses the newly chosen input files. Reflected captions never select those files or authorize access to them.

The shared diff engine receives the actual Base/Mine/Theirs encodings when reading working files, repository stages, rebound sources or patch recovery. This preserves the source's Blame conversion decisions after Reload and Open, without keeping a stale BOM/encoding mask from a previous document. Original text remains available for source display and ordinary saved content.

The launch session follows its currently adopted controller. A discarded original Git result still fails the original command, even if the replacement editor closes cleanly. Retired reads and exports drain before session completion. The new native continuity fixture performs an actual three-file → two-file → three-file handoff in one native window, checks reflected names versus input URLs, rejects readonly Save, preserves all input/output bytes and waits for current/retired work to finish. Its existence alone is not a passing checkpoint.

Batch evidence records the combined checkpoint with Blame matching, reflected patch exports and standalone presentation. Physical focus/VoiceOver, complete displayed source parity, reflected-path EditorConfig semantics, signed sandbox terminal authorization and minimum-OS/ Intel acceptance remain separate work. This review certifies no whole dialog or whole-app completion.