Equality-constrained least-squares solver

[loiseaujc]

Following #1044, this PR implements an equality-constrained least-squares solver based on xGGLSE from lapack.

Proposed interfaces

• call solve_constrained_lstsq(A, b, C, d, x [, storage, overwrite_matrices, err])
• x = constrained_lstsq(A, b, C, d [, overwrite_matrices, err])

where A and b define the least-squares cost, while Cx = d define the equality constraints.

Progress

• Interface
• Base implementation
• Tests
• In-code documentation
• Specifications
• Example

Ping: @perazz, @jvdp1, @jalvesz

[loiseaujc]

Unless I forgot something, I think this PR is ready to be reviewed :)

[Copilot]

Pull Request Overview

This PR adds support for equality-constrained least-squares solving to the stdlib_linalg module. It introduces functionality to solve problems of the form: minimize ||Ax - b||² subject to Cx = d, using LAPACK's GGLSE backend.

Key changes:

• New constrained least-squares solver with both function and subroutine interfaces
• Support for workspace pre-allocation via constrained_lstsq_space
• Comprehensive test coverage and documentation examples

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/stdlib_linalg.fypp	Adds public interfaces for constrained_lstsq, solve_constrained_lstsq, and constrained_lstsq_space
src/stdlib_linalg_least_squares.fypp	Implements constrained least-squares solver, fixes typo "byu" → "by", adds check_problem_size validation
src/lapack/stdlib_linalg_lapack_aux.fypp	Adds handle_gglse_info error handler for GGLSE LAPACK routines
test/linalg/test_linalg_constrained_lstsq.fypp	Adds comprehensive test suite covering function, subroutine, and pre-allocated workspace interfaces
test/linalg/CMakeLists.txt	Registers new test file in build system
example/linalg/example_constrained_lstsq1.f90	Demonstrates functional interface usage
example/linalg/example_constrained_lstsq2.f90	Demonstrates subroutine interface with pre-allocated workspace
example/linalg/CMakeLists.txt	Registers example programs in build system
doc/specs/stdlib_linalg.md	Adds documentation for all three new interfaces

Comments suppressed due to low confidence (2)

doc/specs/stdlib_linalg.md:1

• Corrected spelling of 'GLSE' to 'GGLSE' to match the actual LAPACK routine names.

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doc/specs/stdlib_linalg.md:1

• Corrected spelling of 'GLSE' to 'GGLSE' to match the actual LAPACK routine names.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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