147 destructure field must exist in payload

// Pins that a binding-position destructure (`| ok { <name> }`) validates its
// field names against the branch's payload shape at the KORU level. Here `ok`
// carries `{ a: i32, b: u8 }`, so `{ nonexistent }` names no field — it must be
// a koru diagnostic, NOT a raw host-level Zig `no field named` leak from
// generated code. Position is what makes `{ ... }` a destructure: a destructure
// happens at BINDING, never in the event declaration.
~import std/io

~event read { path: []const u8 }
| ok { a: i32, b: u8 }
| err []const u8

~read => ok { a: 1, b: 2 }

~read(path: "x")
| ok { nonexistent } |> std/io:print.ln("{{ nonexistent:d }}")
| err _ |> _