RISC-V: Initialise Buddy branch correctly

Closes RV-568

Stacked MRs

• RISC-V: Initialise Buddy branch correctly (!17570 - merged) (you are here)
• RISC-V: Abolish `allocate` on `Layouts` (!17566 - merged)
• RISC-V: Prefer `new` over `allocate` with `bind` (!17569 - merged)
• RISC-V: New state allocation and initialisation... (!17567 - merged)

What

Uses the new allocation+initialisation mechanism introduced in !17567 (merged). This allows us to remove the deep_refresh helper method which was previously required in the absence of a "full" allocation (this has the same effect as deep_refresh).

Why

This achieves correctness in the "correct by construction" sense. In other words, you create a new state, it is properly initialised and there are no pitfalls with regards to its use.

Manually Testing

cargo test --lib buddy

Benchmarking

Runtime performance is unchanged.

Tasks for the Author

• Link all Linear issues related to this MR using magic words (e.g. part of, relates to, closes).
• Eliminate dead code and other spurious artefacts introduced in your changes.
• Document new public functions, methods and types.
• Make sure the documentation for updated functions, methods, and types is correct.
• Add tests for bugs that have been fixed.
• Put in reasonable effort to ensure that CI will pass.
  • make -C src/riscv
  • dune build src/lib_riscv
  • dune build src/rust_deps
• Benchmark performance and populate the table above if needed.
• Explain changes to regression test captures when applicable.
• Write commit messages to reflect the changes they're about.
• Self-review your changes to ensure they are high-quality.
• Complete all of the above before assigning this MR to reviewers.