Add README for corpus integrity tests

Co-authored-by: bact <128572+bact@users.noreply.github.com>

Author: Copilot

tests/corpus_integrity/README.md

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+# Corpus Integrity Tests
+
+This directory contains tests that verify the integrity, format, and parseability of corpus files in PyThaiNLP.
+
+## Purpose
+
+These tests are separate from regular unit tests because:
+1. They test actual file loading and parsing (not mocked)
+2. Downloadable corpus tests require network access and can be slow
+3. They verify corpus data format and structure
+4. They should only run when corpus files or corpus code changes
+
+## Test Categories
+
+### Built-in Corpus Tests (`test_builtin_corpus.py`)
+
+Tests corpus files that are included in the package:
+- Text word lists (negations, stopwords, syllables, words, etc.)
+- CSV files (provinces)
+- Frequency data (TNC, TTC)
+- Name lists (family names, person names)
+
+**Run time:** < 1 second
+
+### Downloadable Corpus Tests (`test_downloadable_corpus.py`)
+
+Tests corpus files that need to be downloaded:
+- OSCAR word frequencies (96MB)
+- TNC bigram/trigram frequencies (41MB + 145MB)
+
+**Run time:** ~17 seconds (includes download time)
+
+## Running Tests
+
+### Run all corpus integrity tests:
+```bash
+python -m unittest discover -s tests/corpus_integrity -v
+```
+
+### Run only built-in corpus tests:
+```bash
+python -m unittest tests.corpus_integrity.test_builtin_corpus -v
+```
+
+### Run only downloadable corpus tests:
+```bash
+python -m unittest tests.corpus_integrity.test_downloadable_corpus -v
+```
+
+## CI Integration
+
+The corpus integrity tests run automatically via GitHub Actions workflow (`.github/workflows/corpus-integrity.yml`) when:
+- Changes are made to `pythainlp/corpus/**`
+- Changes are made to `tests/corpus_integrity/**`
+- The workflow file itself is modified
+
+## What is Tested
+
+Each test verifies:
+1. **Loadability**: File can be loaded without errors
+2. **Type correctness**: Returns expected data type (frozenset, list, dict)
+3. **Non-empty**: Contains actual data
+4. **Format validity**: Data structure matches expected format
+5. **Content validity**: Contains expected content (e.g., Thai characters)
+
+## Adding New Tests
+
+When adding a new corpus file or function to `pythainlp.corpus`:
+1. Add a test to `test_builtin_corpus.py` if it's included in the package
+2. Add a test to `test_downloadable_corpus.py` if it requires download
+3. Verify the test catches format errors by temporarily breaking the corpus
+
+## Relationship to Unit Tests
+
+- **Unit tests** (`tests/core/test_corpus.py`): Use mocks for speed, test code logic
+- **Corpus integrity tests** (this directory): Use real data, test file integrity
+
+Both test suites are important and complementary.