017 destructure nested host

// Nested destructure — including INTO A HOST TYPE. The emitter only writes
// access paths (payload.pos.x); Zig validates them at the next stage, so
// interiors can be fully opaque to Koru. This is the scaling pin: the
// checker polices names and linearity, the host polices types.
~import std/io

const Pos = struct { x: i64, y: i64 };

~pub event locate {}
| at { pos: Pos, label: []const u8 }

~proc locate|zig { return .{ .at = .{ .pos = .{ .x = 3, .y = 7 }, .label = "home" } }; }

~locate()
| at { pos: { x, y }, label } |> std/io:print.ln("{{ label:s }} at {{ x:d }},{{ y:d }}")