subflow variant with continuations

// Test 8215: Subflow variant with continuations.
//
// Shape: `~event|variant = invocation(args) | branch binding |> result_constructor`
// Combines the variant-arm declaration (new, established by 8213/8214) with
// the subflow-with-continuations shape from 302_subflow_flow (grounded passing test).
//
// Currently expected to trip the @compileError marker in visitor_emitter.zig
// because the variant emission loop only handles continuations.len == 0.

~import std/io
~import std/build

const std = @import("std");

~event greet { value: i32 }
| final i32

// Two helper events with proc bodies, copied from 302_subflow_flow.
~event double { value: i32 }
| result i32

~proc double|zig {
    return .{ .result = value * 2 };
}

~event identity { value: i32 }
| result i32

~proc identity|zig {
    return .{ .result = value };
}

// Default unvariant arm — passes value through.
~greet = identity(value)
| result r => final r

// Variant arm — doubles. Same shape as the default arm but |two-tagged.
~greet|two = double(value)
| result r => final r

// Register the |two variant.
~std/build:variants {
    "input:greet": "two"
}
| configured _ |> _
| skipped _ |> _
| invalid-event _ |> _

~event print-result { value: i32 }

~proc print-result|zig {
    std.debug.print("{}\n", .{value});
}

// Expected: greet(value: 21) picks |two variant → double(21) → 42 → print 42.
~greet(value: 21)
| final f |> print-result(value: f)