test(learn): cover the rest of the non-interactive core

[Chemaclass]

🤔 Background

src/learn.sh is 1213 lines with 6 tests over 4 functions. That gap is why the
duplication cleanup pass declined to touch it — roughly 50 lines of real, repeated code
sit in there, and a refactor had nothing to catch a mistake.

💡 Changes

• Cover print_menu, show_progress, reset_progress, create_example_file and run_lesson_test (both the pass and fail paths): 4 functions → 9, 6 tests → 14
• Verified non-vacuous by mutation: dropping the rm in reset_progress and the chmod in create_example_file each fail their test, and restoring returns to green

🔍 What the sandbox needed

Three things worth recording, since each looked like a test bug at first:

• stdin from /dev/null so the Press Enter to continue prompts hit EOF instead of blocking — plus || true at the call sites, because that read returns non-zero and the sandbox runs under set -e
• BASHUNIT_ROOT_DIR set inside the child shell (run_lesson_test shells out through it): it is readonly in the caller, so a VAR=x cmd prefix assignment aborts with readonly variable
• the colour palette stubbed empty — learn.sh renders colour but does not source colors.sh, and pulling in the real chain (str → globals → env → colors) would couple a unit test to source order