Define data structure for methods

We need a data structure to represent a method, including signature info and a sequence of basic blocks. This is the foundation for adding control-flow later.

Scope

Create method model with:

• Visibility (public/private)
• Name
• Return type
• Parameters (name + type)
• A list of basic blocks.

Define a basic block model with:

• Stable identifier (e.g. id or label)
• List<String> for the instructions

Place implementation in core.

Notes

• No control-flow semantics required in this issue
• Keep it easy to extend with branches later