chore: migrate motoko/parallel_calls to icp-cli

[marc0olo]

Summary

• Replace dfx.json with icp.yaml using @dfinity/motoko@v5.0.0 recipe for both backend (caller) and callee canisters
• Rename source directories: src/caller/ → backend/, src/callee/ → callee/ with entry points app.mo
• Update mops.toml to moc 1.9.0, core 2.5.0, add --default-persistent-actors flag
• Remove redundant persistent keyword from actor declarations (covered by --default-persistent-actors)
• Rewrite Makefile with numbered tests using icp canister call (calls setup_callee, then tests both sequential and parallel endpoints)
• Update README.md to canonical icp-cli structure with correct prerequisites and deploy instructions
• Add CI workflow .github/workflows/parallel_calls.yml (motoko job only — Rust not yet migrated)
• Delete dfx.json and rust-toolchain.toml

Test plan

• icp network start -d && icp deploy deploys both backend and callee canisters successfully
• make test runs all 5 tests:
  1. setup_callee links the callee principal to the backend canister
  2. sequential_calls(100) returns 100
  3. parallel_calls(100) returns 100
  4. sequential_calls(2000) returns 2000
  5. parallel_calls(2000) returns a value (fewer than 2000 due to in-flight limit)
• CI workflow passes on the PR

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