fix(features): deterministic, symbol-aware IV estimate (#9)

[bradsmithmba]

Summary

ImpliedVolatilityEstimator had two compounding defects in src/features/greeks.py:

1. Non-deterministic. estimate_iv() multiplied a per-category base volatility by a random factor seeded on wall-clock time:

   random.seed(int(time.time()) % 100)
   adjustment = random.uniform(0.8, 1.2)
   return base_vol * adjustment

   The same symbol returned different IVs on consecutive calls, so no Greek or probability-of-profit derived from it was reproducible across a run.

2. SPY hardcoded. get_iv_for_position() called estimate_iv("SPY") unconditionally, applying a low-vol ETF base (0.15) to every position regardless of the real underlying — systematically underestimating IV for individual names.

Closes #9.

Fix

• Remove the time-seeded random factor; estimate_iv() returns the deterministic per-category base (ETF 0.15 / large-cap tech 0.25 / general 0.30).
• Add an optional symbol argument to get_iv_for_position(position, symbol=None). When the underlying is known it flows through; when unknown, fall back to the general single-stock base (0.30) instead of assuming SPY. (Position carries no symbol field today, which is why the method had nothing to pass — the new parameter lets callers supply it.)

Verification

Exercised directly (the test file can't be collected on master yet — see below):

estimate_iv('AAPL') x2 -> 0.25, 0.25   (deterministic)
estimate_iv('SPY')     -> 0.15
estimate_iv('XYZ')     -> 0.30
position IV, no symbol -> 0.30 base (was 0.15 SPY)
position IV, 'AAPL'    -> 0.25 base   (symbol now matters)

A determinism test (test_iv_estimation_is_deterministic) is added to tests/test_greeks.py.

CI note

tests/test_greeks.py imports src.features.greeks, which triggers src/features/__init__.py → the data layer missing until #1 (PR #19) merges, so the file can't be collected on master yet. The fix was verified by loading the module directly (it depends only on position_models, numpy, and scipy). The new test runs in CI once #1 lands.

🤖 Generated with Claude Code

[bradsmithmba]

Merge-ordering note: the determinism test added here lives in tests/test_greeks.py, which imports src.features.greeks → src/features/__init__.py → the data layer missing until #1 (PR #19) merges, so the file can't be collected on master yet. Recommend merging #19 first; after that the test runs and passes. The fix was verified by loading the module directly (it depends only on position_models, numpy, scipy).