Support nested flexible array members

[jsgf]

Add support for structs where the last field is itself a struct containing a flexible array member (FAM). For example:

struct Name {
    int count;
    char name[];
};

struct Field {
    int id;
    struct Name name;  // Last field has a FAM
};

The Name struct is generated with a generic FAM parameter:

pub struct Name<FAM: ?Sized = [c_char; 0]> {
    pub count: c_int,
    pub name: FAM,
}

And Field propagates this parameter to its nested field:

pub struct Field<FAM: ?Sized = [c_char; 0]> {
    pub id: c_int,
    pub name: Name<FAM>,
}

For packed structs, the nested FAM field is wrapped in ManuallyDrop since packed structs cannot directly contain DST fields:

#[repr(C, packed)]
pub struct FieldPacked<FAM: ?Sized = [c_char; 0]> {
    pub id: c_int,
    pub name: ::std::mem::ManuallyDrop<Name<FAM>>,
}

Implementation:

• Enhanced flex_array_member() in ir/comp.rs to recursively detect when a struct's last field is a compound type with its own FAM
• Updated field code generation to parameterize nested FAM field types with and wrap in ManuallyDrop for packed structs
• Added test cases for nested FAMs in both regular and packed structs

Related to #2936