release(v1.1.0): additive CPU-only generic numerical primitives

Adds 9 new top-level / sub-modules to the Rust API only (no Python
bindings yet), with at least one analytic unit test per module.

New Rust modules:
- optimal_control::matrix_riccati  (RK4 backward solver)
- timeseries_utils::nonsync_covariance  (Hayashi-Yoshida)
- timeseries_utils::wavelet  (Haar / Daubechies DWT and MODWT)
- risk_measures  (VaR, CVaR, projected sub-gradient CVaR minimisation)
- graph::laplacian + graph::spectral_clustering  (Jacobi + k-means++)
- topology  (Vietoris-Rips persistent homology, bottleneck distance)
- volterra  (Caputo Adams, Markovian lift, second-kind Volterra,
             Fourier inversion of characteristic functions)
- signatures  (truncated tensor signature, log-sig, random reservoir,
               Salvi-Cass-Lyons signature kernel, shuffle product)

All previously stable APIs untouched; abi3-py38 ABI preserved.
New module tests: 29/29 passing. Pre-existing 5 unrelated failures
unchanged.

docs/source/algorithms/signatures.rst

@@ -0,0 +1,119 @@
+Path Signatures
+===============
+
+The module :code:`signatures` provides truncated tensor signatures
+(Lyons 1998), log-signatures, random reservoir projections, and the
+Salvi--Cass--Lyons signature kernel.
+
+Truncated Signature
+-------------------
+
+For a continuous path :math:`X : [0, T] \to \mathbb{R}^d` of bounded
+variation, the *signature* is the formal series
+
+.. math::
+
+   S(X)_{0,T}
+   \;=\;
+   1 + \sum_{k \ge 1} \sum_{i_1, \dots, i_k}
+       S^{i_1, \dots, i_k}_{0, T}\,
+       e_{i_1} \otimes \dots \otimes e_{i_k},
+
+with iterated Stieltjes integrals
+
+.. math::
+
+   S^{i_1, \dots, i_k}_{0, T}
+   \;=\;
+   \int_{0 < u_1 < \dots < u_k < T}
+       dX^{i_1}_{u_1}\, \dots\, dX^{i_k}_{u_k}.
+
+For piecewise-linear input with increments :math:`\Delta_n`, the
+truncated signature obeys the multiplicative recursion
+
+.. math::
+
+   S^{(M)}_{0, t_n}
+   \;=\;
+   S^{(M)}_{0, t_{n-1}}\,\otimes_M\,\exp_M(\Delta_n),
+
+where :math:`\exp_M(\Delta) = \sum_{k=0}^M \Delta^{\otimes k} / k!`.
+
+Log-Signature
+-------------
+
+The truncated tensor logarithm
+
+.. math::
+
+   \log(S)
+   \;=\;
+   \sum_{n \ge 1} \frac{(-1)^{n+1}}{n}\,(S - 1)^{\otimes n}
+
+lives in the truncated free Lie algebra and provides a more
+parsimonious representation.
+
+Random Signature
+----------------
+
+Following Cuchiero--Schmocker--Teichmann (2023), one drives a random
+reservoir on :math:`\mathbb{R}^N`,
+
+.. math::
+
+   dZ_t = A_0 Z_t\, dt + \sum_{i=1}^d A_i Z_t\, dX^i_t,
+
+with random matrices :math:`A_i \in \mathbb{R}^{N \times N}` whose
+entries are i.i.d. Gaussian with variance :math:`1/N`. The map
+:math:`X \mapsto Z_T` is a finite-dimensional random projection of
+:math:`S(X)`.
+
+Signature Kernel (Salvi--Cass--Lyons)
+-------------------------------------
+
+The signature inner product
+
+.. math::
+
+   K(s, t) \;=\; \langle S(X)_{0, s},\; S(Y)_{0, t}\rangle
+
+solves the linear hyperbolic PDE
+
+.. math::
+
+   \frac{\partial^2 K}{\partial s\,\partial t}
+   \;=\;
+   \langle \dot X_s, \dot Y_t \rangle\, K(s, t),
+   \qquad
+   K(s, 0) = K(0, t) = 1.
+
+It is integrated on a uniform grid via the Goursat scheme
+
+.. math::
+
+   K_{i+1, j+1}
+   = K_{i+1, j} + K_{i, j+1} - K_{i, j}
+     + \langle \Delta x_i, \Delta y_j\rangle\,
+       \tfrac{1}{2}(K_{i+1, j} + K_{i, j+1}).
+
+API
+---
+
+.. code-block:: rust
+
+   pub struct TruncatedSignature {
+       pub channels: usize,
+       pub level: usize,
+       pub tensors: Vec<Vec<f64>>,
+   }
+   pub fn path_signature(path: &[Vec<f64>], level: usize) -> Result<TruncatedSignature>;
+   pub fn log_signature(sig: &TruncatedSignature) -> Result<TruncatedLogSignature>;
+
+   pub struct RandomSignatureConfig {
+       pub reservoir_dim: usize, pub seed: u64, pub variance: f64,
+   }
+   pub fn random_signature(path: &[Vec<f64>], cfg: &RandomSignatureConfig)
+       -> Result<RandomSignatureResult>;
+
+   pub fn signature_kernel(x: &[Vec<f64>], y: &[Vec<f64>])
+       -> Result<SignatureKernelResult>;