QL: guard source-only non-axisymmetric execution

[krystophny]

Closes #122. This child is stacked on #119, #121, and #124 and remains open against main.

Summary

• reject a fileless non-axisymmetric solve when neither a perturbation file nor an active, nonzero helical drive supplies the source
• use the production helical-source predicate shared with the drive implementation
• cover rejected zero-source and accepted driven-source configurations through the public initialization and validation path

Invariants

The guard runs before Arnoldi assembly. It does not change the numerical algorithm, discretization, convergence criteria, generated code, ABI, storage layout, Fourier convention, CGS units, source signs, boundary conditions, or switch-off response. A valid nonzero helical source passes unchanged; an identically zero source terminates before zero-norm normalization can create NaNs.

No branch in this stack is authorized to merge before the programme-wide gate.

Verification

Driven case fails on parent

The two process-level tests were applied without the production change at parent 7a868ae:

$ ctest --test-dir build -R '^ntv_(zero|driven)_source_guard_test$' --output-on-failure
50% tests passed, 1 tests failed out of 2
ntv_zero_source_guard_test   Passed
ntv_driven_source_guard_test Failed
ERROR: non-axisymmetric solve has no source.

The parent incorrectly rejected isw_hel_drive=1 with nonzero hel_phim_re.

Both cases pass after fix

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

$ make test
100% tests passed, 0 tests failed out of 9

$ ctest --test-dir build -R '^ntv_(zero|driven)_source_guard_test$' --output-on-failure
100% tests passed, 0 tests failed out of 2

The zero-source test emitted the single expected diagnostic and returned in under five seconds. The driven test emitted Driven source accepted!. Neither output contained NaN or zhseqr.

$ git diff --check
<no output>

The 16 separately managed external-data regressions remain registered under regression;external-data; this branch does not mark that unavailable-data gate passed.