v0.2.1 — the help text was still lying

--help was still lying. 0.2.0 implemented both of this site's
captchas and corrected the README — but not the engines' own text, which is
what a user reads first.

Two engines were still telling readers "NO challenge has ever been observed
on this site … neither setting has anything to act on today, and neither
helps with a refusal". Playwright's said Cloudflare's challenge "is reported
as blocked instead, so no solve is attempted or billed for it". Both describe
a repo that stopped existing in 0.2.0.

Fixed

• Every user-facing claim that a captcha cannot be solved is gone — from
  --help, from four docstrings and from page_flow.block_advice. What
  replaced them is what was measured: reCAPTCHA Enterprise ~55s /
  $0.00299, Turnstile 11s / $0.00145, and PerimeterX's widget-less stub
  as the one refusal nothing can buy.

• --help named a host the parser refuses. .co.th was dropped in 0.1.0
  — it redirects to robinhood.co.th, a different company — and HOSTS has
  held ten entries ever since, while three engines and output_writer.py
  went on saying "eleven country sites" and playwright's --url help listed
  .co.th among the supported ones.

• The README contradicted itself about Cloudflare. One section said a
  managed challenge is what a non-browser client gets and that a browser
  engine seeing one means "something has stripped the context" — three
  sections above the Turnstile section explaining that foodpanda.com serves
  one to a real browser, which is how this repo solved one. Same
  contradiction in TROUBLESHOOTING.md.

Added

• A flag reference in the README. There was none: sixteen real flags were
  documented nowhere but --help. Four tables — core, proxies, 2Captcha, and
  the engine differences — with defaults and what each flag actually does.

• Three checks, because prose does not fail a build on its own: a
  banned-phrase check over every shipped .py and .md; a check that the
  documented host count equals len(HOSTS) and that no user-facing text
  advertises a refused host; and a check that every flag an engine accepts
  appears in the README, that every in-page link resolves, and that each exit
  code is documented.

  Each was verified to fail when its fix is reverted. A check that cannot
  fail is not a check.

532 offline checks, up from 493.