Prevent ENCODE starch files from being misrouted into the BED→tabix pipeline and poisoning the cache — Closes #69

[conradbzura]

Summary

Stop ENCODE starch files from poisoning the content-addressed cache. ENCODE labels BEDOPS starch archives as BED upstream, so the tabix processor ran zcat -f | sort | bgzip on them; because starch is neither gzip nor text, the pipeline produced a valid BGZF wrapping scrambled bytes and committed it before tabix failed, leaving a corrupt data artifact that later reads served and every retry re-failed against.

Add a byte-sniff guard in TabixIntervalProcessor that inspects the downloaded source's leading bytes and fails before any cache write when they are neither gzip nor plain text. The guard is format-agnostic (it catches starch and any other surprise binary), needs no new system dependencies, and leaves the ontology mapping and routing untouched. bigBed is exempt because its binary source is handled by bigBedToBed. Real starch support via the BEDOPS toolchain is intentionally out of scope and left as a follow-up.

Closes #69

Proposed changes

Reject non-gzip/non-text sources before caching (src/cfdb/workflows/processors/tabix.py)

• Add _source_looks_processable(prefix): accept a gzip/bgzip magic prefix or a NUL-free prefix that decodes as UTF-8 (trimming up to three trailing bytes so a multi-byte character split at the sniff boundary is not falsely rejected); treat an empty prefix as acceptable so the existing zero-data-line guard still owns the empty-source case.
• Add _read_prefix to read the leading bytes and _assert_source_processable to raise a clear RuntimeError naming the format, local id, and observed magic when the source is unsupported.
• Invoke the guard in _stage_prepare immediately after download_source and before the pipeline (hence before any cache.put), for every format except bigBed.

Cover the guard (tests/test_workflows/test_processors_tabix.py)

• Add predicate tests (examples plus two Hypothesis properties) and two run()-level behavior tests.

Test cases

#	Test Suite	Given	When	Then	Coverage Target
1	_source_looks_processable	A gzip/bgzip magic prefix	The predicate is called	It returns True	Gzip acceptance
2	_source_looks_processable	A plain BED text prefix	The predicate is called	It returns True	Text acceptance
3	_source_looks_processable	A BEDOPS starch magic prefix	The predicate is called	It returns False	Starch rejection
4	_source_looks_processable	A NUL-containing binary prefix	The predicate is called	It returns False	Binary rejection
5	_source_looks_processable	A short non-text prefix shorter than the trim window	The predicate is called	It returns False	Trim-to-empty branch
6	_source_looks_processable	UTF-8 text whose final multi-byte char is cut at the boundary	The predicate is called	It returns True	Boundary-split recovery
7	_source_looks_processable	An empty prefix	The predicate is called	It returns True	Empty deferred to downstream guard
8	_source_looks_processable	Any bytes appended to the gzip magic	The predicate is called	It always returns True	Property: gzip prefix accepted
9	_source_looks_processable	Any bytes appended to the starch magic	The predicate is called	It always returns False	Property: starch prefix rejected
10	TestTabixIntervalProcessorPipelines	A BED file whose source is a starch archive	run is awaited	It raises and commits no cache artifact	Fail-before-commit on starch
11	TestTabixIntervalProcessorPipelines	A bigBed file with a binary source	run is awaited	It completes and commits the artifact	bigBed guard exemption