Add initial control-flow graph model and Prism builder

[Morriar]

Why

Rubydex needs a control-flow graph before it can model instruction semantics, local-variable flow, reachability, and type inference.

This PR introduces the minimum CFG topology needed to start that work. The representation follows Sorbet’s non-SSA CFG structure but does not attempt full Sorbet compatibility yet.

What changed

Added a CFG model with:

• Stable BlockIds
• Dedicated entry and exit blocks
• Ordered instructions within each BasicBlock
• Jump, Branch, and Exit terminators
• Source locations for instructions and control-flow transfers
• Successor traversal and block lookup APIs

Added a Prism visitor that builds CFGs for:

• Sequential statements
• if and unless
• while and until
• return
• Parenthesized expressions
• Nested branching predicates

The builder preserves Ruby evaluation order and creates explicit branch, merge, loop-header, loop-exit, and unreachable blocks. These structural blocks may be empty; a future CFG simplification pass can remove unnecessary forwarding blocks.

Instructions currently contain only source locations. Instruction semantics, locals, block arguments, and type information will be introduced separately.

Testing

Builder tests render the complete CFG into a canonical textual representation. Each test verifies:

• Block allocation
• Instruction ordering
• True and false successors
• Merge edges
• Loop backedges
• Return-to-exit edges
• Nested control flow

Follow-up work

This PR intentionally does not include:

• Instruction lowering
• Local variables or temporary values
• break, next, redo, or retry
• Exception, rescue, or ensure control flow
• Method arguments or default arguments
• CFG simplification
• Sorbet fixture compatibility tests