test: add text-cleaner corpus and close gaps surfaced by it

The 21-fixture corpus (test-cases/text-cleaner-corpus/) exercises the cleaner
end-to-end against the spec in TEST-CASES.md. Closing the failing cases drove
five small cleaner fixes plus two fixture-generation fixes:

- _SMART_CHARS: add prime, double prime, guillemets (case 03)
- _ZERO_WIDTH: add soft hyphen U+00AD (case 05)
- clean_dataframe: clean column headers via the same pipeline (cases 16/19/20),
  with a clean_headers toggle on CleanOptions
- smart_title_case: title-case full-shout strings ("ALICE SMITH" -> "Alice
  Smith") while still preserving embedded acronyms; preserve uppercase after
  apostrophe in names ("O'CONNOR" -> "O'Connor", "o'neil" -> "O'neil")
- test_corpus.py reader: pre-strip NUL bytes (C parser truncates at NUL,
  python engine is too strict about embedded literal "), per spec case 06
- generate_test_data.py: properly CSV-escape literal-quote cells in case 03
  expected; quote the rogue-comma price field in case 17 input

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

test-cases/text-cleaner-corpus/expected/20_kitchen_sink.csv

@@ -0,0 +1,5 @@
+id,Name,"""Email""",Notes
+1,Alice Smith,Alice@Example.COM,"""VIP"" customer - contact ASAP..."
+2,Bob Jones,bob@example.com,"it's 5'6"" tall"
+3,Carol Brown,CAROL@EXAMPLE.COM,3 × 4 = 12 (preserve ×)
+4,,empty@example.com,whitespace-only name (becomes empty)