086 accept when only optional effect branch

// ============================================================================
// Test 210_086: POSITIVE — optional effect branch covered only by a
//                         `when`-guarded handler is FINE.
//
// Counterpart to 210_085. The required-branch coverage rule does NOT
// apply to optional `!` branches: optional means "no handler is a legal
// trajectory," so a `when`-guarded handler whose false case produces no
// work is just exercising that contract.
//
// `! ?warning w when w.len > 0 |> ...` alone covers the only thing the
// consumer cares about (non-empty warnings); empty-warning fires are
// silently dropped, which is exactly what the `?` decl promised.
//
// Expected: compiles cleanly. (No runtime assertion needed — we're
// pinning that the validator does NOT reject this shape.)
// ============================================================================

~import "$std/io"

~pub event tokenize { source: []const u8 }
! token []const u8
! ?warning []const u8
| done usize

~proc tokenize|zig {
    token("t1");
    warning("");           // empty — guard will skip
    token("t2");
    warning("real");       // non-empty — guard will match
    return .{ .done = 2 };
}

~tokenize(source: "x")
! token t |> std.io:print.blk {
    token {{ t:s }}
}
! warning w when w.len > 0 |> std.io:print.blk {
    warn {{ w:s }}
}
| done n |> std.io:print.blk {
    done {{ n:d }}
}