chore: migrate rust/icp_transfer to icp-cli

[marc0olo]

Summary

• Replaces dfx.json with icp.yaml using @dfinity/rust@v3.3.0
• Moves canister source from src/icp_transfer_backend/ to backend/, renames Cargo package to backend
• Adds Makefile with 4 end-to-end tests: ledger balance check, fund backend canister, backend.transfer call, post-transfer balance check
• The make test target deploys the ICP ledger locally at ryjl3-tyaaa-aaaaa-aaaba-cai (same principal as mainnet) via provisional_create_canister_with_cycles, matching the hardcoded MAINNET_LEDGER_CANISTER_ID in the Rust code
• Adds CI workflow .github/workflows/icp_transfer.yml using ghcr.io/dfinity/icp-dev-env-rust:1.0.0
• Deletes legacy rust-icp-transfer-example.yml, dfx.json, and demo.sh

Test plan

• icp network start -d && icp deploy && make test passes locally
• CI workflow rust-icp_transfer job passes
• Test 1: default identity has 100 ICP initial balance
• Test 2: fund backend canister with 1 ICP succeeds (ledger returns Ok)
• Test 3: backend.transfer of 0.5 ICP returns Ok with a block index
• Test 4: backend ledger balance decreased after transfer

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