Add DAPHNE ETH peak descriptor support

[marroyav]

Description

No parent issue linked yet.

This PR adds DAPHNE Ethernet peak descriptor support to DAPHNEEthFrame for the
FDDAQ v5.5.0 target environment.

The change adds typed access to the DAPHNE Ethernet peak descriptor words,
exposes the new descriptor accessors through Python bindings, and adds a
DAPHNEEthFrame_test unit test covering ADC, header, peak descriptor, and
packed-word roundtrip behavior.

Target environment:

• FDDAQ release/work area: fddaq-v5.5.0-a9-1
• Package base tag: fddaq-v5.5.0 / v2.6.0
• Tested on: np04-srv-017

Dependencies:

• No upstream package dependency is required for this PR.
• The matching rawdatautils branch
  marroyav/daphneeth-fddaq-v5_5_0 depends on this dataformat API.

For the paired end-to-end decode smoke test, also build the matching
rawdatautils branch and run:

python sourcecode/rawdatautils/scripts/daphne_decoder.py \
  /nfs/home/marroyav/waffles/scripts/np02vd_raw_run043950_0000_df-s03-d1_dw_0_20260417T125036.hdf5.copied \
  --det VD_CathodePDS -n 1

Type of change

• Documentation (non-breaking change that adds or improves the documentation)
• New feature or enhancement (non-breaking change which adds functionality)
• Optimization (non-breaking change that improves code/performance)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (whatever its nature)

Testing checklist

• Unit tests pass (e.g. dbt-build --unittest)
• Minimal system quicktest passes (pytest -s minimal_system_quick_test.py)
• Full set of integration tests pass (dunedaq_integtest_bundle.sh)
• Python tests pass if applicable (e.g. python -m pytest)
• Pre-commit hooks run successfully if applicable (e.g. pre-commit run --all-files)

Testing performed on np04-srv-017:

• dbt-build -c -j4 passed.
• dbt-build --unittest fddetdataformats passed.
• DAPHNEEthFrame_test passed as part of the package unit tests.
• Paired decode smoke test with rawdatautils passed on run 43950,
  VD_CathodePDS, record (1, 0):
  • source ID 800: 1407 DAPHNEEth frames
  • source ID 801: 1395 DAPHNEEth frames

Minimal system quicktest, full integration tests, separate Python pytest, and
pre-commit were not run.

Further checks

• Code is commented where needed, particularly in hard-to-understand areas
• Code style is correct (dbt-build --lint, and/or see https://dune-daq-sw.readthedocs.io/en/latest/packages/styleguide/)
• If applicable, new tests have been added or an issue has been opened to tackle that in the future.
  (Indicate issue here: # (issue))

dbt-build --lint was not run. New unit coverage was added in
unittest/DAPHNEEthFrame_test.cxx.