[Roadmap] DSL extension at the abstract/semantic level

## Objective

Extend the DSL at the abstract (semantic) level to support new problem formulations and transformations.

## Scope

- DSL transformations for multi-phase problems, free final time, interior point constraints
- Penalization (à la Malisani)
- Symbolic reduction (with ModelingToolkit.jl integration)
- Work at the abstract/semantic level (IR) rather than the parsing level

## Why

Extending the DSL at the semantic level (via the IR being introduced in the CTParser refactoring) will make it easier to add new features without modifying the parser. This is closely tied to the CTParser IR work ([CTParser #289](https://github.com/control-toolbox/CTParser.jl/issues/289)).

## References

- [CTParser roadmap discussion](https://github.com/control-toolbox/CTParser.jl/discussions/288)
- [CTParser #289 — IR refactoring](https://github.com/control-toolbox/CTParser.jl/issues/289)
- [OptimalControl #658 — Integrate ModelingToolkit](https://github.com/control-toolbox/OptimalControl.jl/issues/658)
- [Roadmap comment](https://github.com/orgs/control-toolbox/discussions/68#discussioncomment-17584843)