A camera topic mixing image formats is still filed as an infrastructure failure #331

What happens

_decode_camera_images (src/hflow/transform.py:509) refuses a camera topic whose messages
carry more than one image format:

if len(formats) != 1:
    raise ValueError(f"camera topic {topic!r} mixes image formats {sorted(formats)}")
return images, frame_ids, _input_codec_for_image_format(formats.pop(), topic)

The very next line calls _input_codec_for_image_format, which raises SourceNotConforming
after #288. So two refusals about the same channel's contents, one line apart, land in
different ingest_failures.failure_kind values:

camera topic '/cam' mixes image formats ['jpeg', 'png']  -> infrastructure
camera topic '/cam' carries unsupported format 'bogus'   -> source-unsupported

Why it matters

infrastructure is the kind an operator retries. A recording whose camera topic mixes jpeg and
png fails identically on every retry, which is exactly the reasoning in #203 that produced
source-unsupported in the first place. #288 scoped itself to the four refusals #203 listed,
and this one was not on that list, so nothing here is a regression. It is the one place the new
vocabulary is inconsistent with itself.

Definition of done

_decode_camera_images raises SourceNotConforming for the mixed-format case, and a test
triggers the real raise site (not a synthetic exception) and asserts
classify_ingest_failure(error) == IngestFailureKind.SOURCE_UNSUPPORTED. tests/test_ingest_ledger.py
has four of these to copy. The message text does not need to change.

Check whether the docstring list in SourceNotConforming (transform.py:114) and the paragraph
in docs/how-to/write-a-converter.md enumerate the refusal categories, and add this one if they
do.

Validation

uv sync --locked --all-extras
uv run ruff check --fix
uv run ruff format
uv run ty check
uv run pytest -q

Run plain uv run pytest -q from the repo root: pytest tests alone misses
packages/hflow-server/tests.

Notes

Found while reviewing #288.