feat : [linalg] add bi-conjugate gradient stabilized method

[jalvesz]

This PR co-developed with CoPilot introduces the Bi-Conjugate Gradient stabilized method. It is inspired from SciPy's implementation https://github.com/scipy/scipy/blob/v1.16.2/scipy/sparse/linalg/_isolve/iterative.py#L158-L304 but following the internal framework introduced in #994

Key interfaces :

• call stdlib_solve_bicgstab_kernel (A, M, b, x, rtol, atol, maxiter, workspace)
• call stdlib_solve_bicgstab(A, b, x [, di, rtol, atol, maxiter, restart, precond, M, workspace])

• Implementation
• Documentation
• unit testing (symmetric and non-symmetric)
• examples ( 2 : dense and CSR )

[Copilot]

Pull Request Overview

This PR introduces the Bi-Conjugate Gradient Stabilized (BiCGSTAB) method for solving non-symmetric linear systems. The implementation follows the internal framework established in #994 and is inspired by SciPy's BiCGSTAB implementation.

Key changes:

• Adds BiCGSTAB kernel and user-friendly interface for dense and CSR matrices
• Moves preconditioner enums to main module for shared use across solvers
• Includes comprehensive test coverage and example programs

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/stdlib_linalg_iterative_solvers_bicgstab.fypp	Core BiCGSTAB implementation with kernel and interface procedures
src/stdlib_linalg_iterative_solvers.fypp	Main module updated with BiCGSTAB interfaces and shared preconditioner enums
src/stdlib_linalg_iterative_solvers_pcg.fypp	Removes duplicate preconditioner enum definitions
test/linalg/test_linalg_solve_iterative.fypp	Adds comprehensive test cases for BiCGSTAB method
example/linalg/example_solve_bicgstab.f90	Basic BiCGSTAB usage example
example/linalg/example_solve_bicgstab_wilkinson.f90	Advanced example using Wilkinson matrix with CSR format
src/CMakeLists.txt	Adds new BiCGSTAB source file to build
example/linalg/CMakeLists.txt	Adds BiCGSTAB examples to build
doc/specs/stdlib_linalg_iterative_solvers.md	Documentation for BiCGSTAB interfaces and usage

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[jvdp1]

LGTM! Thank you @jalvesz

[jalvesz]

Thanks for your reviews @jvdp1, I'll merge this one which I think is ready. I just opened an issue to don't forget about the coo2csr discussion.