InputMapper tracks a mouse-down over a placed piece and promotes it to
drag mode once the cursor travels past an 8px threshold. Legal drop
cells (those where the piece's start→end vector still fits a legal
placement) are highlighted in green. Releasing on a legal cell emits a
RelocateRequested signal; Main feeds it to MovePieceCommand, which is
already undoable via the existing history stack.
Escape or releasing on an invalid cell cancels. The harness gains a
relocate() helper so UI tests can script drag-and-drop moves without
synthesizing motion events.
