docs(validation): document the fault status in the how-to

[simontaurus]

Follow-up to #151, which I should have included there.

docs/how-to/validation.md still told the reader "Exit code is 0 only when no check failed". A fault also makes it non-zero, and the how-to never mentioned the status at all - docs/architecture.md was the only place it was described.

The addition says what a fault means for someone reading CLI output: it is our bug rather than a finding, it carries the id of the check that broke, it is counted separately, it costs one check rather than the run, and seeing one means filing a bug here instead of editing their schema.

make docs-test clean.

[github-actions]

Release preview

No version bump from the current commits (stays at v0.20.0). Use conventional commit types (feat, fix, ...) to trigger a release.

Changelog preview (truncated)

_{Preview via python-semantic-release and conventional commits.}

[github-actions]

📊 Benchmark Results

Click to see benchmark comparison

📊 Benchmark Comparison (threshold: 1.3x)
============================================================

➖ Unchanged (within threshold):
  ➖ test_simple_dict_document_store: 0.0012s → 0.0012s (+0.5%)
  ➖ test_sqlite_document_store: 0.0013s → 0.0013s (+1.1%)
  ➖ test_local_sparql_store: 0.0289s → 0.0287s (-0.6%)
  ➖ test_oneof_subschema: 0.0452s → 0.0450s (-0.5%)
  ➖ test_enum_docstrings: 0.0378s → 0.0402s (+6.3%)
  ➖ test_subclass_inheritance: 0.0417s → 0.0444s (+6.5%)
  ➖ test_class_hierarchy: 0.0382s → 0.0471s (+23.4%)
  ➖ test_core[v1]: 0.0285s → 0.0285s (+0.2%)
  ➖ test_core[v2]: 0.0324s → 0.0328s (+1.4%)
  ➖ test_schema_generation[v1]: 0.0012s → 0.0012s (+1.5%)
  ➖ test_schema_generation[v2]: 0.0020s → 0.0020s (-0.1%)
  ➖ test_simple_json: 0.0004s → 0.0005s (+8.0%)
  ➖ test_complex_graph: 0.0011s → 0.0011s (+0.3%)

============================================================
Summary: 0 regressions, 0 improvements, 13 unchanged
============================================================

✅ No significant performance regressions

Threshold: 1.3x (30% slower triggers a regression warning)

Note: Benchmarks are informational only and won't fail the build.

_{💡 Tip: Download the benchmark-results artifact for detailed JSON data}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!