GDCMSeriesFileNames: support gantry-tilt CT ordering by default (follow-up to #6468)

Follow-up to #6468 (now closed): the `GDCMSeriesFileNames` migration onto `gdcm::IPPSorter` landed (`50dfb136658`, `7cb3bbec23d`), but @dzenanz's specific request from that discussion is not yet fully satisfied and deserves its own tracking issue.

> Gantry-tilt CTs should continue to be supported. I encountered this with pediatric patients - technicians were trying hard to avoid the eyes while scanning.

## Current state on `main`

`gdcm::IPPSorter` treats gantry tilt as an error, so with the default `m_FailOnAmbiguousOrdering = true` (`itkGDCMSeriesFileNames.h:258`) a gantry-tilt series now **throws** instead of ordering. Gantry-tilt CT is only handled by setting `FailOnAmbiguousOrdering` to false, which routes to the ported SerieHelper fallback — and the code comment for that path (`itkGDCMSeriesFileNames.cxx:234`) calls it "an untrustworthy, non-standard hack whose output should not be trusted."

So the acquisition @dzenanz named — real, clinically-motivated pediatric head CT — is supported only via an opt-in path the code itself disavows, and not at all by default.

## The ask

Decide and implement first-class gantry-tilt handling for `GDCMSeriesFileNames`. Options to weigh:

1. Detect gantry tilt explicitly (non-orthogonal slice-normal vs. orientation) and order it correctly via IPP-projected-on-normal, *without* routing through the disavowed legacy fallback — i.e. make the tilt case trustworthy rather than merely tolerated.
2. Keep the strict default but promote gantry tilt out of the "ambiguous → throw" bucket into a supported, documented, correctly-sorted case.
3. Confirm whether `IPPSorter`'s own `SetComputeZSpacing` / tilt handling can be leaned on before falling back to ported logic.

Whatever the choice, the acceptance test is a gantry-tilt CT series that sorts correctly under the **default** settings, with a regression test fixture.

## Related

- #6468 — the discussion this splits from (closed; concrete migration deliverable landed)
- #6467 — the migration PR
- #2735, #6464, #6466 — DCMTK IPP-on-normal ordering parity

Read itkGDCMSeriesFileNames.h around m_FailOnAmbiguousOrdering and itkGDCMSeriesFileNames.cxx around the SerieHelper fallback, then inspect gdcm::IPPSorter and the ordering discussion in #6468. Determine how gantry tilt should be handled without the disavowed fallback. Done means a gantry-tilt CT fixture sorts correctly with default settings and a regression test covers it.

Written by the indexing model from the issue text.