v0.2.0

Behaviour changes:

• A page buys at most one paid CAPTCHA solve, however many times it is
  fetched. A page re-fetched after a refusal used to start a new
  allowance: 2 solves per page with no proxy pool, and one per exit with a
  pool (4 on a pool of four). With TWOCAPTCHA_KEY set, that was money.
• A served page that states rows and parses to none is now a parser
  failure (stop_reason: parser_found_nothing): exit 5 on page 1
  (was 4, "the listing is empty") and exit 6 on a later page (was 0 and
  status: complete).
• CSV cells that start with =, +, -, @, a tab or a line break
  get a leading ', so a spreadsheet does not run them. JSON is
  unchanged; the sidecar's csv_cells_escaped counts the difference.
• New usage errors (exit 2): --retries 0; --pages,
  --concurrency and --max-books below 1; a negative --delay,
  --retry-delay or --proxy-block-retries; a negative --retries,
  --pages below 1 or a negative delay in scraper_api_client.py. If a
  script of yours passes --retries 0, pass 1: the flag counts
  ATTEMPTS, so 1 means "try once and do not retry".

Fixed

• --retries 0 fetched nothing and called it a result. The load loop is
  range(1, retries + 1), so 0 never runs it and the engine judged
  about:blank. Three third-party audits of this family ran exactly that and
  reported a working scraper as broken. --retries now goes through
  page_flow.attempts in every engine; a check reads the source of each
  one and calls the validator, so a new engine declaring the flag with a
  plain int fails the suite.
• The paid-solve limit was per call, not per page. SOLVES_PER_PAGE = 1
  was applied inside each navigation, and every re-fetch after a refusal (a
  fresh browser, or the next exit in a pool) navigated again. One
  allowance now lives for the whole page, is checked before each purchase
  and is not reset by a retry or a rotation; the language check that lands
  on the book before page 1 of a reviews run spends from page 1's. The
  suite's old assertion solves <= 2 pinned the bug; it now asserts 1, with
  no pool, with a pool of four, and on a reviews landing.
• A page whose own count says it holds rows, parsed to none, was reported
  as an empty listing or as the end of one. It is now
  parser_found_nothing, the page is saved for an issue, and the canary
  names it.
• Output writes are atomic. JSON, CSV and the sidecar are written to a
  temporary file in the same directory and renamed over the target, so a
  crash or a full disk mid-write leaves the previous file whole (before: a
  539-byte out.json came back as 10 bytes). A new file gets the mode
  open() would give it under the current umask; an existing file keeps its
  own.
• --delay, --retry-delay, --pages, --concurrency,
  --proxy-block-retries and --max-books are validated in every engine,
  and --pages, --delay, --retries and --retry-delay in
  scraper_api_client.py (where --retries counts EXTRA attempts, so 0
  stays legal there).
• Comments, docstrings and one --concurrency error message pointed at
  notes that are not public; each rule is now stated where it is used. The
  captcha module's notes described another site; they describe this one.
• The expired-endpoint hint no longer says a Scraping Browser profile lasts
  "about a day": one was measured working after ten days, so the HTTP 401 is
  the signal, not an age.