Add option to use Visual Studio built-in diff for Diff Against Have Revision

[rtsummit]

Summary

• Adds a new Use Visual Studio Diff option (Tools > Options > P4EditVS, default on) that opens "Diff Against Have Revision" inside Visual Studio using IVsDifferenceService instead of launching p4vc diffhave in P4V.
• The have revision is fetched via p4 print -q -o <temp> into %TEMP%\P4EditVS_<guid>_have<ext>, preserving the file extension so VS picks the right content type.
• Existing p4vc diffhave behaviour is preserved — flip the option off to restore it.

Why

Keeps diff inside the IDE so the workflow doesn't break to switch focus to P4V. The original p4vc diffhave path is unchanged behind the toggle for users who prefer it.

Implementation notes

• VSDIFFOPT_LeftFileIsTemporary alone does not reliably remove the temp file, and per-frame close hooks race with the diff editor still holding the file handle. Temp paths are instead tracked in a static list and cleaned up on P4EditVS.Dispose.
• All existing Runner / OutputWindow / status bar conventions are reused — p4 print failures show up in the P4EditVS output pane and as a status bar failure, identical to other commands.
• No new SDK dependencies (IVsDifferenceService / SVsDifferenceService are present in both the VS2019 and VS2022 SDKs used by this project).

Test plan

• VS2022 flavor builds clean (Release / Any CPU) — no warnings or errors.
• Option "Use Visual Studio Diff" appears in Tools > Options > P4EditVS > Settings, defaults to on.
• With the option on, Diff Against Have Revision opens VS's built-in diff window with <filename>#have on the left and the workspace file on the right.
• After closing Visual Studio, %TEMP%\P4EditVS_* files are removed.
• With the option off, the existing p4vc diffhave codepath runs unchanged (note: p4vc diffhave behaviour itself is outside the scope of this PR).
• p4 print failure (e.g. file with no have revision) surfaces in the P4EditVS output pane and as a status bar failure — needs verification.
• VS2019 flavor — relies on CI build; not built locally for this PR.

🤖 Generated with Claude Code