Extensions API

Extension is a named bundle of optional profile contributions.

Method	Return values
`directives()`	`DirectiveSpec` values
`roles()`	`RoleSpec` values
`directiveHandlers()`	`DirectiveHandler` values
`roleHandlers()`	`RoleHandler` values
`lintRules()`	context, document, or source rules
`fixPasses()`	`FixPass` values
`formatterPasses()`	`FormatterPass` values
`statisticsProviders()`	`StatisticsProvider` values

AbstractExtension supplies empty implementations so an extension overrides only the contracts it owns. Profile::withExtension() returns a new profile.

Handler contexts preserve the active renderer or converter state. Source passes return patches; statistics return namespaced scalar values. Extensions cannot activate raw HTML or file reads without the corresponding application policy.

See Extension points and Custom extension.