chore: migrate rust/parallel_calls to icp-cli

[marc0olo]

Summary

• Replaces dfx.json with icp.yaml using @dfinity/rust@v3.3.0 for both caller and callee canisters
• Moves sources from src/caller/ and src/callee/ to top-level caller/ and callee/ subdirs, workspace Cargo.toml updated accordingly
• Eliminates setup_callee — the caller now discovers the callee principal at first use via PUBLIC_CANISTER_ID:callee injected by icp-cli
• Upgrades ic-cdk from 0.17 to 0.20 and updates inter-canister calls to the new Call::bounded_wait API (replacing the removed ic_cdk::api::call::call)
• Updates rust-toolchain.toml to remove pinned 1.78.0 channel (too old for current dependencies)
• Adds Makefile with 4 numbered tests covering sequential/parallel behavior at both small (n=10) and large (n=2000) call counts
• Adds rust-parallel_calls and rust-parallel_calls-multi-subnet jobs to the existing parallel_calls.yml CI workflow; deletes the old rust-parallel-calls-example.yml
• The multi_subnet/ PocketIC test is preserved unchanged; make test-multi-subnet builds WASMs via cargo build and runs the cross-subnet timing test

Test plan

• icp network start -d && icp deploy && make test passes in rust/parallel_calls
• make test-multi-subnet passes (requires PocketIC server installed)
• CI jobs rust-parallel_calls and rust-parallel_calls-multi-subnet pass in the workflow
• Existing motoko-parallel_calls CI jobs are unaffected

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