chore: migrate motoko/icrc2-swap to icp-cli

[marc0olo]

Summary

Migrates motoko/icrc2-swap from dfx to icp-cli.

Originally contributed by 0xAegir.

Code:

• backend/app.mo: converted from actor class Swap(init_args) to plain actor Swap; token principals read from PUBLIC_CANISTER_ID:token_a / PUBLIC_CANISTER_ID:token_b via Runtime.envVar<system> — icp-cli injects them during deploy; no init args needed
• Security improvements (from canister-security skill review):
  • withdraw: added try/catch around icrc1_transfer — if the token ledger traps, the balance is refunded and #CallFailed returned; previously the balance was permanently debited
  • withdraw: reject anonymous principal (uses msg.caller for balance lookup)
  • swap: check both users have non-zero balances before swapping; returns #InsufficientBalance instead of silently moving zero tokens
• ICRC.mo: inline ICRC-1/2 type definitions kept for educational visibility

Deploy (make deploy):

• Creates icrc2-alice / icrc2-bob identities with --storage plaintext (compatible with CI containers that have no system keyring)
• Deploys token_a pre-funded for icrc2-alice, token_b pre-funded for icrc2-bob
• Deploys backend with --mode reinstall -y (works on fresh and existing deployments)
• No --args needed for backend — it discovers token principals via env vars

Tests (make test, 7 tests, idempotent):

1. Empty balances on fresh deployment
2. Swap with no deposits returns InsufficientBalance
3. Alice approves + deposits token_a (approve = amount + fee)
4. Bob approves + deposits token_b
5. Swap 1:1
6. Alice withdraws token_b; verifies ledger balance delta = 99_990_000
7. Bob withdraws token_a; verifies ledger balance delta = 99_990_000

README:

• Leads with the two key safety patterns (debit-before-transfer, atomic swap)
• Fee handling section explaining approve/withdraw arithmetic
• #CallFailed guidance: retry; if persistent, contact token ledger operator
• Known limitations accurately describe async edge cases
• Links to ICRC-2 digital asset standards and inter-canister calls security best practices

Test plan

• mops check passes
• CI motoko-icrc2-swap job passes all 7 tests