Add unit tests for the four-way OI flow classifier

[Abhay512]

The classifier in backend/engine/classifier.py maps an OI change plus the simultaneous price change onto a build-up type, and flows_4way() in backend/engine/ranking.py folds those into bullish/bearish rupee totals. Both are pure functions, both are central to every board, and neither has direct test coverage today.

What to do

Add backend/tests/test_classifier.py covering every branch:

OI	price	expected
up	up	long build-up
up	down	short build-up
down	up	short covering
down	down	long unwinding

Then cover flows_4way(): build a StockRank with known values and assert the bullish and bearish totals. backend/tests/test_oneway.py has a _rank() helper you can copy.

Acceptance

• Every classification branch asserted, including the zero-change edge case
• flows_4way() asserted on at least one hand-computed example
• Runs with python -m tests.test_classifier, no market or API key needed
• ruff check . clean

Getting started

pip install -r requirements.txt and python -m tests.test_oneway to confirm your setup. No broker subscription required.

[utkarshalpha]

I have a tested implementation covering all four classifier branches, zero-change neutrality, and hand-computed four-way flow totals. I'll open the PR shortly.