chore: migrate rust/performance_counters to icp-cli

[marc0olo]

Summary

• Migrate rust/performance_counters from dfx to icp-cli (icp.yaml, @dfinity/rust@v3.3.0)
• Move Rust source from src/lib.rs to backend/lib.rs; add workspace Cargo.toml with members = ["backend"]
• Update inter-canister call API from deprecated ic_cdk::call() to ic_cdk::call::Call and ic_cdk::api::canister_self() (ic-cdk 0.20)
• Add Makefile with make test asserting non-zero counters for for_update, for_composite_query, and example
• Add CI workflow (.github/workflows/performance_counters.yml) with ghcr.io/dfinity/icp-dev-env-rust:1.0.0
• Delete dfx.json, BUILD.md, performance_counters.did, .devcontainer/

Test plan

• CI passes
• icp network start -d && icp deploy && make test succeeds locally