ci: add PR gate workflow, structure/rules checker, and algorithm tests

[utk2103]

What this adds

A GitHub Actions workflow that runs on every PR against main, so review starts from a green or red signal instead of a manual read-through.

.github/workflows/pr-check.yml — three steps: the gate, compileall scripts, then unittest discover tests.

.github/check_pr.py — diffs the PR against its base branch and enforces the parts of CONTRIBUTING.md that can be checked mechanically.

Hard failures:

• New top-level entry outside the whitelist, or any move/rename/delete of a tracked file
• A subdirectory or wrong extension inside scripts/, notebooks/, docs/, tests/
• A third-party import that isn't in requirements.txt (with an alias map for cv2, PIL, bs4, yaml, sklearn, dotenv)
• A script that moves or deletes files with no --dry-run flag
• A notebook whose saved output contains a local path like /Users/...

Warnings only (non-blocking):

• A script with real logic and no assert and no test naming it
• More than 1 MB of saved notebook output

The two warning rules are deliberately not errors: four scripts already on main break them, and blocking beginners on pre-existing debt is the wrong trade. Flip them to errors once that backlog is clear.

Run it locally the same way CI does:

python .github/check_pr.py --base main
python .github/check_pr.py --selftest   # covers the parsing helpers

tests/test_algorithms.py — 12 unittest cases for the pure functions in the search and sort scripts: binarySearch (found / missing / empty), linearSearch (found / first match / missing), mergeSort (in place, duplicates and negatives, 0–2 elements), twoSum (pair indices, unsorted input, no pair). They load scripts by path because 01.TwoSum.py isn't a valid module name, and suppress stdout because two of the scripts print at import.

check.py, toh.py, multithreading.py and qr_code_generator.py are untested here — the first three call input() or sleep at import time, so they need refactoring into functions first. Separate PR.

Drive-by fix

requirements.txt was corrupted on main: the qrcode[pil] line had been appended as UTF-16LE into a UTF-8 file, putting a NUL byte after every character. pip install -r requirements.txt fails on it, and the gate can't read the file either. Stripped the NULs; the line now reads normally.

Verification

$ python -m unittest discover tests
Ran 35 tests in 0.003s
OK

$ python .github/check_pr.py --selftest
selftest ok

$ python .github/check_pr.py --base origin/main
4 changed file(s), 0 error(s), 0 warning(s).

The gate was also dry-run against every existing file in scripts/ and tests/: 0 errors, 4 warnings (the untested scripts named above).

🤖 Generated with Claude Code

[utk2103]

Value add imo