refactor: remove finance-specific code from point_processes module

- Removed order_flow.rs (UnifiedTheoryParams, OrderFlowAnalyzer, MarketImpact)
- Removed analyze_order_flow, unified_theory_params, market_impact Python bindings
- Updated mod.rs documentation to be generic (no trading references)
- Kept general-purpose: Hawkes, fBM, mfBM, Mittag-Leffler, Hurst estimation

Finance-specific code moved to rust-hft-arbitrage-lab-internal

docs/source/api/hmm.md.backup

@@ -0,0 +1,16 @@
+# API: HMM
+
+```python
+from optimizr import HMM
+
+model = HMM(n_states=2)
+model.fit(X, n_iterations=100, tolerance=1e-6)
+states = model.predict(X)
+logp = model.score(X)
+```
+
+Parameters
+- `n_states`: number of hidden regimes
+- `fit(X, n_iterations=100, tolerance=1e-6)`: train with Baum-Welch
+- `predict(X)`: Viterbi decoding → `np.ndarray`
+- `score(X)`: log-likelihood