Trace periodic join compatibility and nullspace

[krystophny]

join_ends replaces one periodic equation per retained Laguerre block with the Lorentz current-closure constraint. The remaining periodic seam could therefore have come from an incompatible source, an incorrect nullspace, or reconstruction of a different source.

Add an opt-in NEO2_JOIN_END_TRACE_FILE diagnostic. For each join it records the Lorentz source correction, transfer-matrix identity error, original periodic-system left and right null vectors, compatibility residual, both replaced-row residuals, and algebraic scales. The CSV writer rejects wrong shapes, nonfinite values, nonpositive scales, and I/O failures.

The diagnostic is observation-only. When disabled it allocates no diagnostic arrays and performs no diagnostic solve. It does not change the Lorentz projection, Pfirsch-Schlueter current constraint, particle-conservation treatment, collision operator, discretization, convergence criteria, Fourier convention, CGS units, boundary conditions, ABI, or array layout.

The accepted 480-step, 30-pitch batch replay resolves the open hypothesis:

• periodic compatibility residual: 2.81e-17 relative;
• replaced p/m row residuals: 1.18e-14 and 1.17e-14 relative;
• replaced-row difference: 1.39e-16 relative;
• left/right null residuals: 1.39e-16 and 5.27e-15 relative;
• Lorentz projection identity residual: 2.71e-16 relative;
• Lorentz source correction: 7.85e-4 of the unsigned source scale;
• periodic reconstructed-distribution seam: 7.28e-4;
• periodic current-contraction seam: 8.48e-4.

The periodic solve is compatible and satisfies both original rows. The seam instead follows from applying the Lorentz correction only to the joined propagator source. Stage-1 reconstruction replays the saved local sources, which do not contain a spatially localized form of that correction. This PR does not invent such a localization and does not accept a physical pointwise current.

With the diagnostic enabled, all 398 saved propagators and 199 reconstructed boundaries are bit-for-bit identical to the parent result. Every numerical dataset in the 199 local-response HDF5 files and the global response file is also identical. All 12 focused tests and the trace-analysis test pass under bounds checks and floating-point traps. A follow-up commit separates finiteness and positivity checks because Fortran logical operators are not required to short-circuit; this prevents a nonfinite diagnostic scale from reaching an ordered comparison under invalid-operation traps.

Verification

Test fails on parent

At 130ea2c:

rm -f build/TEST/join_end_trace.csv
fo test join_failure_diagnostic_test
test -s build/TEST/join_end_trace.csv

join_failure_diagnostic_test PASS
focused_test_status=0
trace_file_status=1

The existing test passes, but no requested join-end trace is produced.

Test passes after fix

rm -f build/TEST/join_end_trace.csv
fo test join_failure_diagnostic_test
test -s build/TEST/join_end_trace.csv

join_failure_diagnostic_test PASS
focused_test_status=0
trace_file_status=0

fo

Static: OK
Build: OK
Tests: OK
Lint: OK
Fmt: WARN
All stages passed

The format warning is already present for the legacy join_ends.f90 file at the parent commit. This PR keeps its existing layout instead of reformatting unrelated lines.