refactor: agree on a common name for batched MPC operations

[crStiv]

• Standardized all batched MPC operations to use _many suffix
• Renamed core functions: mul_vec→mul_many, local_mul_vec→local_mul_many, etc.
• Updated trait definitions and all implementations across the codebase
• Updated all function calls and test cases
• Ensures consistent API naming across rep3, shamir, and plain protocols

Changes affect:

• Core arithmetic operations (mpc-core)
• MPC trait definitions (co-noir/common)
• Specialized implementations (co-circom, co-groth16, co-plonk)
• All test cases and benchmarks

Closes #351

[HusseinAdeiza]

Hi @crStiv! I see this PR has been open for 10 months with merge conflicts.

I'd be happy to help resolve the conflicts and get this merged. I did an analysis (see my comment on #351) and agree with standardizing on _many.

Would you like me to:

1. Rebase your branch and fix conflicts, OR
2. Create a fresh PR with the same changes?

Let me know! Happy to collaborate. 🚀