fix(grading): stop autograder from silently crashing under bash 5

[lassebenni]

Summary

• ((var += expr)) reports a failing exit status whenever the resulting arithmetic value is 0 -- a well-known bash gotcha. Combined with set -euo pipefail, the grader silently dies mid-run the moment a student's running score hits exactly 0 at a checkpoint (e.g. a mostly-empty early submission).
• This only shows up on GitHub Actions' Ubuntu runners (bash 5+); it's invisible when testing locally on macOS (bash 3.2 ships without this strictness), which is how it went unnoticed.
• Also guards bare check_*() helper calls (which legitimately return 1 to signal warn/fail, not a script error) with || true so they don't trip set -e either.

Test plan

• bash -n .hyf/test.sh
• Reproduced against an empty scaffold repo with GNU bash 5: exit code 1 before the fix, exit code 0 after.
• Re-ran against a known-good local solution: score/pass output unchanged (regression-free).
• Confirm a real PR run in CI now completes the "Run tests" step for a low/zero-scoring submission.

🤖 Generated with Claude Code

[github-actions]

📝 HackYourFuture auto grade

Assignment Score: 47 / 100 ❌

Status: ❌ Not passed
Minimum score to pass: 60
🧪 The auto grade is experimental and still being improved

Test Details

=== Week 9 Autograder ===
  ✓ PASS  found validation_queries.sql
  ✓ PASS  found schema_setup.sql
  ✓ PASS  found data_dictionary.md
  ✓ PASS  found verification_results.sql
  ✓ PASS  found AI_ASSIST.md
  ✓ PASS  Level 1: required files (10/10 pts)
  ✗ FAIL  validation_queries.sql: file is empty or still contains unfilled TODO stubs
  ✓ PASS  Level 2: Task 1 validation queries (0/20 pts)
  ✓ PASS  schema_setup.sql: file filled (no stub TODOs)
  ✓ PASS  schema_setup.sql: vw_dim_zones view defined
  ✓ PASS  schema_setup.sql: vw_fact_trips view defined
  ✓ PASS  schema_setup.sql: negative fare filter (fare_amount >= 0) present in vw_fact_trips
  ✗ FAIL  schema_setup.sql: vw_fact_trips must cast pickup_datetime as TIMESTAMP (pickup_datetime::TIMESTAMP) — required for time-pattern queries in Task 4 (Task 2)
  ✓ PASS  Level 3: Task 2 star schema views (22/30 pts)
  ✓ PASS  data_dictionary.md: file filled (no stub TODOs)
  ✓ PASS  data_dictionary.md: 'Grain' heading/label present
  ✓ PASS  data_dictionary.md: primary key documented
  ✓ PASS  data_dictionary.md: measures list includes at least one aggregatable column
  ✓ PASS  Level 4: Task 3 data dictionary (15/15 pts)
  ✗ FAIL  verification_results.sql: file is empty or still contains unfilled TODO stubs
  ✓ PASS  Level 5: Task 4 verification queries (0/15 pts)
  ✗ FAIL  screenshot missing: /home/runner/work/c55-data-week-9/c55-data-week-9/assignment/assets/borough_count.png not found
  ✓ PASS  Level 6: borough screenshot (0/5 pts)
  ✗ FAIL  AI_ASSIST.md: file is empty or still contains unfilled TODO stubs
  ✓ PASS  Level 7: Task 5 AI log (0/5 pts)

Score: 47 / 100  (passing: 60)  pass=false