229 nested union shared arm capture

// Nested unions sharing an arm name with different payload-ness.
//
// Reported by an external user 2026-10-31: `u1.same` is payload-free and
// `u2.same` carries a string. The emitSubflowContinuationsWithDepth
// return-switch path forwarded the chain head's event name into nested
// switches, so the inner `| same g` arm's payload check read `u1.same`
// (payload-free) and dropped the `|g|` capture — undeclared identifier
// in the emitted Zig. `.other` survived only because `u1` has no `other`
// arm and the lookup's not-found default is "has payload".
//
// Expected: `u2.same`'s payload binds `g`, frees it, prints `ok`.

~import std/io

~pub tor u1 { } | same | diff

~proc u1|zig {
    return .{ .same = .{} };
}

~pub tor u2 { }
| same string<std/io:allocated!>
| other string<std/io:allocated!>

~proc u2|zig {
    return .{ .same = koru_allocator().dupe(u8, "payload") catch @panic("oom") };
}

~pub tor go { }
| ok
| err string<std/io:allocated!>

~go = u1()
| diff => ok
| same |> u2()
    | same g |> std/io:free(content: g) => ok
    | other g |> std/io:free(content: g) => ok

~go()
| ok |> std/io:print.ln("ok")
| err e |> std/io:print.ln("err")