0.5.1

Wave C of docs/spec/16: indexnowkit/sitemap goes back to suggest in the adapters (symfony-bundle 0.6.0, laravel
0.7.0, yii2 0.4.0). The core gains the two pieces an adapter needs to keep an optional package optional; nothing changes
for applications.

Added

• Check\StaticCheck(CheckLevel $level, string $line): one fixed line of check, decided when the adapter is
  wired — sitemap: not installed (composer require indexnowkit/sitemap) when the optional package is absent.
• Adapter\ConfigFactory(..., array $ignoreBlocks = []): top-level blocks unknownOptions() skips entirely, so a
  configuration written for an optional package does not warn once the package is gone (ignoreBlocks: ['sitemap']).
  Appended with a default: every existing call compiles. Block names only; a dotted key is a LogicException.
• docs/adapters.md §2 "Optional packages": the predicate, the classes behind it, the stub command,
  StaticCheck, ignoreBlocks; §20 lists the same as Definition of Done items.