Route direct file drops through the runtime's QMP event

[btsouth]

Summary

Direct file drops onto the VM window never reached the launcher.

app/file_drop_qemu_windows.go accepted drops with DragAcceptFiles and then tried SetWindowSubclass on QEMU's SDL window to see WM_DROPFILES. QEMU runs in another process, and Windows does not allow subclassing a window across processes: SetWindowSubclass rewrites GWLP_WNDPROC, and a window-procedure pointer is only valid in the process that owns the window. The subclass therefore never ran, and WM_DROPFILES went to SDL, which turns it into SDL_DROPFILE, which QEMU ignored. DragAcceptFiles only sets WS_EX_ACCEPTFILES (a style bit that can be set across processes), which is why it looked enabled and logged success.

The runtime already covers this: runtime-build/patches/qemu/0007-report-native-file-drops.patch emits a DISPLAY_FILE_DROP QMP event that main.go already consumes. This change makes that the only path.

Changes

• Remove the cross-process subclass path (enableVMWindowDrops, vmDropWindowProc, handleVMDrop, dragQueryPoint) and its calls in winapi.go and closeguard.go.
• droppedFilesEvent now also reads an optional drop point (x, y) from the event.
• guestDropPoint scales a display-window point to the guest display; when the event carries no point it uses the cursor, which is still over the release point while QEMU reports the completed drop.
• main.go hands the mapped point to sendDroppedFilesAt, so the guest can deliver into the window under the cursor (guest-side targeting is guest-build/0073).

Notes

• This needs a runtime built with 0007. The shipped winq-emu-alpha10-portable.zip has it; the older C:\WINQ-EMU runtime used in the last laptop pass did not, which is why drops did nothing there.
• Optional follow-up: have 0007 include SDL's drop.x/drop.y in the event so the point is authoritative rather than taken from the cursor. SDL_DropEvent has carried x/y since SDL 2.0.14 and the runtime ships 2.32.

Test plan

• go test ./... (CI: Linux and Windows)
• Windows: drag a file onto the VM window with a runtime that has 0007; the guest should receive it in the window under the cursor and shell.log should stay free of the old drops: lines.

[btsouth]

Thanks. The QMP drop path landed in #137, including the drop-point handling. Closing as superseded.