Add rough Heston stochastic volatility model

[lsbardel]

Introduce a preliminary first-slice RoughHeston model that prices through
the existing characteristic-function pricing pipeline.

The variance is a rough (fractional) square-root process with Hurst
exponent H, so the roughness index is alpha = H + 1/2. Unlike the
classical Heston model, the characteristic function has no closed form: it
is obtained by solving the fractional Riccati equation with a fractional
Adams (Diethelm-Ford-Freed) predictor-corrector scheme, vectorized over
the frequency grid. The model reuses the CIR variance parameters and
follows the same time-changed convention as Heston, so the martingale
convexity correction is applied by the pricing layer. At H = 1/2 it
recovers the classical Heston model.

• quantflow/sp/rough_heston.py: RoughHeston model, fractional Adams solver,
  and an approximate Euler sampler
• quantflow_tests/test_rough_heston.py: characteristic-function properties,
  Heston-limit recovery, Adams convergence, and steeper-skew checks
• docs: API stub, COS pricing example, nav entry, index table row, and
  El Euch-Rosenbaum / Gatheral-Jaisson-Rosenbaum bibliography entries

Calibration, a faster Pade characteristic-function solver, a refined Monte
Carlo scheme, and a full tutorial are deferred to a later phase.

Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_01E2vqE1bifN1kmRboBroqLR

[Copilot]

Pull request overview

This PR introduces a preliminary RoughHeston stochastic-volatility model that plugs into Quantflow’s existing characteristic-function based pricing pipeline, plus documentation and tests to validate key properties and the Heston-limit behavior.

Changes:

• Added RoughHeston model with a fractional Adams (Diethelm-Ford-Freed) solver for the fractional Riccati equation, plus an illustrative Euler-style sampler.
• Added tests covering characteristic-function properties, Heston-limit recovery, solver-step convergence, skew steepness, and sampling sanity.
• Added API docs stub, an example script, mkdocs nav entry, and bibliography references for rough volatility sources.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
quantflow/sp/rough_heston.py	New Rough Heston model implementation and fractional Riccati solver integration for characteristic-function pricing.
quantflow_tests/test_rough_heston.py	New unit tests validating core mathematical and pricing behaviors of the Rough Heston implementation.
mkdocs.yml	Adds Rough Heston API page to documentation navigation.
docs/references.bib	Adds bibliography entries for rough Heston / rough volatility references.
docs/examples/rough_heston_pricer.py	Adds an example comparing short-maturity implied vols vs classical Heston.
docs/bibliography.md	Adds rendered bibliography entries corresponding to the new references.
docs/api/sp/rough_heston.md	Adds mkdocstrings API stub page for RoughHeston.
docs/api/sp/index.md	Adds Rough Heston to the stochastic-process index table.

Comments suppressed due to low confidence (1)

quantflow/sp/rough_heston.py:282

• After applying the t**beta scaling in characteristic_exponent, the inline comment here (“t**beta factor cancels…”) becomes incorrect. Since these weights are computed for h = 1/n (unit interval), the t**beta factor should be applied by the caller.

        h = 1.0 / n  # dt = t / n; the t**beta factor cancels with the psi grid step

[codecov-commenter]

Codecov Report

❌ Patch coverage is 99.03846% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.54%. Comparing base (5e16f37) to head (3aec5cb).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
quantflow/sp/rough_heston.py	99.03%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #87      +/-   ##
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+ Coverage   88.75%   89.54%   +0.78%     
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  Files          84       86       +2     
  Lines        5142     5406     +264     
==========================================
+ Hits         4564     4841     +277     
+ Misses        578      565      -13     

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