ARC advertised "hold Ctrl to switch direction" but the engine never saw modifier
keys: the UI dropped `mods` when building `Input::Point`. PR #407 recorded this as
a known limit.
The UI now writes `Session::ctrl_down` every frame (next to the cursor update, not
only while a command prompts, so previews see it too) and the two ARC paths read it:
- Continue: negating the start tangent makes `arc_sed` pick the mirror arc through
the same two endpoints.
- Center,Start,End: swapping the start/end angles walks the other way around the
same circle; its end-point prompt now carries the same Ctrl hint.
Tests: crates/engine/tests/arc_ctrl_direction.rs.
