test(junk-corpus): pathological-input stress suite for the analyzer

Build a corpus of 35 deliberately-broken files (empty bytes, NUL
bytes, mojibake, UTF-16 without BOM, mismatched columns, unescaped
quotes, corrupt zip, etc.) and pin the analyzer's stability contract
against them.

Files land in ``test-cases/junk-corpus/test_data/``. The generator
``make_junk_corpus.py`` produces them deterministically (one random
sample uses ``secrets.token_bytes`` — committed bytes are stable
across regenerations because the byte stream is captured at commit
time). README documents the categories and how to add new shapes.

``tests/test_junk_corpus.py`` parametrizes over every file in the
corpus and asserts:

1. ``_run_analysis_on_upload`` never raises — exceptions must be
   caught and surfaced as a synthetic ``Finding`` with
   severity="error". This was the user-reported crash for
   13_non_latin_scripts.csv that the previous fix in ae9d4a2
   defensively wrapped; the corpus now stops the regression
   from re-landing on a different shape.
2. Every Finding in the result list is well-formed (string id,
   valid severity, non-empty description).
3. A high-risk subset (empty.csv, only_bom.csv, only_nul.csv,
   corrupt_xlsx.xlsx) MUST surface at least one error-level
   Finding — otherwise the GUI would render "no issues found"
   for a structurally broken file.
4. Error-level Finding descriptions are at least 20 chars so the
   UI banner gives the user something to act on.

Also exclude ``junk-corpus`` from ``tests/test_fixtures_sweep.py``
since that sweep is happy-path (round-trip the text cleaner) and
fights with files designed to break it. The contract is enforced
by the dedicated junk-corpus test, not the sweep.

Runtime: 12 s for the junk-corpus tests, 30 s for the full
project suite (was 19 s without these). 2118 tests pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

test-cases/junk-corpus/test_data/massive_columns.csv

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