6.x: Document attribute-based event listeners - #8345

Documents the attribute-based event listeners introduced in cakephp/cakephp#19469 for CakePHP 6.0.

The Events guide now covers method- and class-level attributes, repeatable subscriptions, callable resolution, priorities, discovery and registration, constructor dependency injection, named event managers, and registration errors and deduplication. Listener registration is organized into listener classes, attribute-based listeners, and anonymous listeners. The Attribute Resolver and 6.0 migration guides link to the new section.

Validation:

• Documentation build, Markdown lint, spelling, internal links, and whitespace checks passed.
• All nine new PHP snippets passed syntax checks and executed in three isolated runtime scenarios, with 19 checks covering discovery, registration, application bootstrap, dependency injection, and named-manager dispatch.
• The feature PR's test suite passed: 24 tests, 40 assertions.
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