Replace GSL fixed-rule quadrature and error handler with fortnum (3/5)

[krystophny]

Merge order

Stack sequence: #29 -> #30 -> #31 -> #32 -> #33.

Each PR is now based on main individually, so its diff is cumulative against main and shares code with its stack neighbors. Merge in the sequence above. Once a predecessor merges into main, the next PR rebases onto it and its diff shrinks to its own increment.

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Third in the fortnum migration stack (base: the hyper1F1 PR). Moves the remaining C-side GSL call sites over, after which no MEPHIT source references GSL.

Dependency removed: GSL fixed-rule Gauss-Legendre quadrature in src/mephit_fem.c (gsl_integration_glfixed_table_*) and the GSL error-handler path in src/mephit_util.c / src/mephit_util.h / src/mephit_run.c (gsl_errno_msg, gsl_set_error_handler, gsl_strerror). The Gauss-Legendre nodes and weights on the unit interval now come from fortnum_gauss_legendre_ab, which folds the table allocation and the [a, b] mapping into one call. fortnum reports failures through return codes rather than a global GSL handler, so the dedicated GSL message hook and its forward declaration are removed.

Verification

Build the shared library at this tip:

$ cmake -S . -B build -G Ninja && cmake --build build --target mephit
[16/19] Building C object CMakeFiles/mephit.dir/src/mephit_util.c.o
[17/19] Building C object CMakeFiles/mephit.dir/src/mephit_fem.c.o
[18/19] Linking CXX shared library lib/libmephit.so

After this PR the library resolves no GSL symbol at all, and fortnum's Gauss-Legendre routine is linked:

$ nm -DC build/lib/libmephit.so | grep -ic gsl
0
$ nm -DC build/lib/libmephit.so | grep -i fortnum_gauss_legendre
0000000000115980 T fortnum_gauss_legendre
00000000001158f0 T fortnum_gauss_legendre_ab

GSL is still on the link line (GSL::gsl) and still searched (find_package(GSL)); both are removed in PR 5 once nothing depends on them. As in earlier PRs, the .x executables link only after PR 4 removes the empty FFTW link.

Note: fortnum pin updated to current main (92de6e9) after a fortnum history rewrite; old shas no longer resolve.