Phase 3: Add logging and automated tests

[ChantelRM]

Suggested pace: ~2 sessions this week

• Add structured log lines to the batch script (timestamp, row counts in/out, pass/fail per stage)
• Install testthat, set up a tests/ directory
• Write tests for validate_schema() (missing column, wrong type)
• Write a test for the Critical_Alert zero-balance rule (the manufactured test row from Issue 10, made permanent)
• Write a test confirming no duplicate Debtor_ID/Record_ID after merge
• Confirm testthat::test_dir("tests") runs clean locally