chore(refactor): update to use new openvm-stark-backend (v2) (#2412)

Fixes imports and usage for v2 `openvm-stark-backend` and
`openvm-cuda-backend`.
So far all positives tests in `crates/circuits/`, `crates/vm/`, and
`extensions/` pass on CPU and GPU. Negative tests need further fixing
(ref INT-5907).

ref INT-4746

---------

Co-authored-by: Claude Opus 4.6 <[email protected]>

Author: jonathanpwang

.gitignore

@@ -2,7 +2,9 @@
 
 # Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
 # More information here https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html
-Cargo.lock
+**/Cargo.lock
+!./Cargo.lock
+.cargo/config.toml
 
 # These are backup files generated by rustfmt
 **/*.rs.bk