032 reject unimplemented event invocation

// Test 020_032: An event with a declared output, INVOKED in the flow, but with
// NO implementation anywhere (no proc, no bare-return impl, no subflow) must be
// a COMPILE ERROR — never a silent zero-default stub.
//
// Today the emitter's !found_impl path (visitor_emitter.zig) synthesizes
// `return .{ .<first-terminal> = 0 }` and the program prints a confident wrong
// answer. That silent fallback is how five AoC honest-red days printed zeros
// instead of failing. The stub is legitimate ONLY where the handler is provably
// never called (template procs, effect-only events, auto-proc passthrough) —
// an invoked event with no implementation is a hole in the program and must
// fail loudly at the koru level.
//
// Grounded on 020_021_event_arrow_return with the `double -> a * 2`
// implementation line deleted.
~import std/io

~pub tor double { a: i32 } -> i32

~double(a: 21): d |> std/io:print.ln("{{ d:d }}")