Feature Request: allow a type to reference a not-yet-declared type (forward reference)

[HansMaulwurf1]

Problem

TRLC requires every type (type/tuple/enum) to be declared before it is referenced. This is explicit, documented language behavior - lrm.trlc.

This makes some perfectly reasonable modeling patterns impossible to express, in particular:

Two (or more) types that reference each other mutually.
Splitting a natural "type + its identifier tuple" pair where the tuple's item field needs to reference the type that uses the tuple (see the mutually-recursive ComponentRequirement/ComponentRequirementId example below).

Minimal reproduction

type Requirement {
    derived_from RequirementId [0 .. *]   // error: unknown symbol RequirementId
}

tuple RequirementId {
    item      Requirement
    separator @
    version   Integer
}

Proposal for a solution

Add an explicit forward-declaration statement that creates the type's identity early, without a body:

forward_declaration ::= 'forward' ( 'type' | 'tuple' | 'enum' ) IDENTIFIER ';'

The later full declaration fills in the previously-created stub instead of registering a new type.

Impact: smaller, self-contained change. The parser stays single-pass; forward becomes a new reserved keyword (low collision risk — not used as an identifier anywhere in the current test suite or LRM examples, but is a breaking change for any external .rsl file that happens to use forward as a name). Still requires new explicit cycle-detection (a completed forward declaration can now form a real cycle, e.g. mutually extends-ing types, or mutually-referencing recursive tuples), and a check that every forward declaration is eventually completed. Existing tests are unaffected since nothing currently uses forward.

Minimal example with forward declaration:

forward tuple RequirementId;

type Requirement {
    derived_from RequirementId [0 .. *]   // now resolves against the forward stub
}

tuple RequirementId {
    item      Requirement
    separator @
    version   Integer
}