QL: allow prescribed perturbation mode without field file

[krystophny]

This branch is stacked on #124 and remains open against main. PR #120 is already merged.

Summary

• add isw_m_phi_input and m_phi_input to ntv_input
• allow file-less non-axisymmetric input with a prescribed toroidal mode
• reject a file-less solve when no physical source is configured
• skip perturbation-file symlinks when no file is requested
• document and test the input path

The #118 drive is the next stack layer. Its #122 child relaxes the no-source guard only when a nonzero helical source exists. No branch is authorized to merge before the programme-wide gate.

Verification

Test fails on main

With only the regression applied:

$ cmake --build --preset default --target test_ntv_perturbation_input
Error: Symbol ‘isw_m_phi_input’ referenced at (1) not found in module ‘ntv_mod’
Error: Symbol ‘m_phi_input’ referenced at (1) not found in module ‘ntv_mod’
Error: Symbol ‘has_perturbation_file’ referenced at (1) not found in module ‘ntv_mod’
ninja: build stopped: subcommand failed.

Test passes after fix

$ fo
Static: OK
Build: OK
Tests: OK
Lint: OK
All stages passed

$ make test
100% tests passed, 0 tests failed out of 6

$ ctest --test-dir build -R "^ntv_perturbation_input_test$" --output-on-failure
1/1 Test #...: ntv_perturbation_input_test ... Passed

$ git diff --check
<no output>

All 16 external-data regressions remain registered under regression;external-data; this branch does not mark that separate gate passed.

[krystophny]

The in_file_pert='none' source-only path this PR adds lets an NA qflux run (isw_qflux_NA=1) proceed with no source configured (no field, drive off). The resulting zero source is the Arnoldi seed, so arnoldi normalizes 0/0 -> NaN and zhseqr fails at every step (~8 min spin, no qflux output). Scoped guard + red-then-green test: #122. Keep arnoldi_mod.f90 untouched; guard the assembled source (a valid none + drive-on run on #119 must still pass).