v0.8.0

Added

• x402 paywall — charge AI agents for a page or an API route without an x402 library:
  P2Flux\Paywall (guard($paymentHeader, $url, $userAgent)), and the two calls under it,
  P2FluxClient::paywallChallenge() and paywallRedeem(). Pay-per-request and prepaid balance; the
  payment is settled before anything is served and serves one response. examples/paywall.php.
• Usage pricing — Paywall::usage($paymentHeader, $url, $maxPrice, $work) and
  P2FluxClient::paywallVerify(): the agent signs for at most $maxPrice (x402 upto), you charge
  what the request cost (paywallRedeem(..., $amount)).
• Requests signed as bots (Web Bot Auth) count as agents under agentsOnly (guard(..., ['signed' => true]));
  an agent's prepaid refund request is answered with its receipt.
  Nothing else changed.