test: single-command runner, cross-platform automation, fixture auto-discovery

Adds a top-level test infrastructure layer addressing four needs at once:
a single command to run anything, cross-platform automation, install/e2e
sanity, and zero-config pickup of new fixtures dropped into test-cases/.

Top-level runner — run_tests.py
  python run_tests.py                # everything (default)
  python run_tests.py --tool dedup   # one tool's tests
  python run_tests.py --unit         # category scopes
  python run_tests.py --e2e          # end-to-end CLI
  python run_tests.py --install      # import / dependency sanity
  python run_tests.py --fixtures     # corpus + dropped-file sweep
  python run_tests.py --coverage     # term-missing report
  python run_tests.py --quick        # skip @pytest.mark.slow
Tools: analyze, cli, config, dedup, io, normalizers, text_clean.

Cross-platform — tox.ini
  Envs for py310-py313 plus install / e2e / fixtures / coverage / lint.
  Forces UTF-8 (PYTHONUTF8=1, PYTHONIOENCODING=utf-8) so identical fixture
  bytes parse the same on Linux/macOS/Windows.

Shared config — pytest.ini
  testpaths, python_files conventions, custom markers (slow, e2e, install,
  fixture_sweep), warning filters that fail on our own DeprecationWarnings
  while tolerating third-party ones.

New test layers
  tests/test_install.py — required deps import; project modules import;
    src.core public API surface; CLI --help exits 0; streamlit app.py
    parses as valid Python; run_tests.py --help works.
  tests/test_e2e.py — CLI roundtrips: cli_analyze table + JSON, cli_text_clean
    --apply writes a real file with NBSP/smart-quote folded, dedup CLI
    removes duplicates, run_tests.py self-tests.
  tests/test_fixtures_sweep.py — parametrizes over every CSV/TSV/XLSX
    inside test-cases/ (excluding text-cleaner-corpus/, which has its own
    suite). Each fixture must: load through repair_bytes, run analyze()
    cleanly, and survive clean_dataframe() with row/col counts unchanged
    plus idempotency. Drop a CSV in, re-run — no test code changes needed.
  tests/test_gap_coverage.py — closes audit gaps: clean_headers=False
    toggle, repair_bytes with tab/semicolon delimiters, BOM+NUL+smart-
    quote combined-fix scenario, analyze() over an XLSX path, sample_rows
    larger than the DataFrame, mid-cell BOM, findings_by_tool edges, plus
    a strict xfail documenting the known §4.17 numeric/phone whitespace
    heuristic gap.

Test count
  Before: 288 passed + 1 xfailed
  After:  475 passed + 2 xfailed (the second xfail is the documented
          collapse_whitespace gap on phone-shaped cells; spec §4.17 calls
          for a heuristic that hasn't been implemented yet).

Functional gaps surfaced (not fixed in this commit):
  - Text cleaner: collapse_whitespace runs unconditionally on every string
    cell, including phone/numeric/date-shaped ones. Spec §4.17 requires a
    skip heuristic. Captured as strict xfail so the gap stays visible.
  - io.read_file does not run pre-parse repair; only analyze() and direct
    callers of read_csv_repaired() get it. CLI tool pages and the dedup
    CLI miss the safety net.
  - Analyzer has no mixed_line_endings detector or near_duplicate_rows
    detector; both planned but require additional plumbing.
  - GUI tool pages each have their own uploader instead of picking up the
    home-page upload through session_state.

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

tox.ini

@@ -0,0 +1,67 @@
+; Cross-platform test automation for DataTools.
+;
+; Drives the pytest suite under multiple Python versions on Linux, macOS,
+; and Windows. Use:
+;
+;   tox                    # all envs
+;   tox -e py312           # one Python version
+;   tox -e e2e             # CLI smoke tests
+;   tox -e install         # import / dependency sanity
+;   tox -e lint            # static checks (mypy / ruff if installed)
+;   tox -e coverage        # full suite with coverage report
+;
+; Adding a new fixture: drop the CSV/XLSX into test-cases/ and re-run.
+; tests/test_fixtures_sweep.py picks new files up automatically.
+
+[tox]
+envlist = py310, py311, py312, py313, install, e2e
+skip_missing_interpreters = true
+isolated_build = false
+
+[testenv]
+description = Run the full pytest suite under {envname}.
+deps =
+    -r requirements.txt
+    -r requirements-dev.txt
+commands =
+    python run_tests.py {posargs}
+passenv =
+    HOME
+    USER
+    LANG
+    LC_ALL
+    PATH
+setenv =
+    PYTHONIOENCODING = utf-8
+    PYTHONUTF8 = 1
+
+[testenv:install]
+description = Verify imports and CLI entry points work after a fresh install.
+commands =
+    python run_tests.py --install -v
+
+[testenv:e2e]
+description = End-to-end CLI smoke tests against real fixtures.
+commands =
+    python run_tests.py --e2e -v
+
+[testenv:fixtures]
+description = Sweep test-cases/ for any newly-dropped fixtures.
+commands =
+    python run_tests.py --fixtures -v
+
+[testenv:coverage]
+description = Full suite with coverage report.
+commands =
+    python run_tests.py --coverage
+
+[testenv:lint]
+description = Static checks (run only if the optional tools are installed).
+deps =
+    -r requirements.txt
+    ruff>=0.5; python_version >= "3.10"
+    mypy>=1.10; python_version >= "3.10"
+allowlist_externals = sh
+commands =
+    sh -c "command -v ruff && ruff check src/ tests/ || echo 'ruff not installed; skipping'"
+    sh -c "command -v mypy && mypy src/ || echo 'mypy not installed; skipping'"