feat: read-only extension points for debugging tools

[ogorzalka]

Five read-only extension points that pollora/debugbar (and any other debugging tool) needs to show what Pollora did during a request. None of them mentions a debugger.

Changes

• Which template answered — FrontendController records a TemplateResolution (template file, conditional tag, Blade view, index fallback, outcome) in the AnsweringTemplate singleton. Requests answered by Route::wp() or a Laravel route leave it empty, which is how a reader tells them apart. Works without WP_DEBUG, unlike TemplateMarker.
• WordPressBooting event — dispatched by Bootstrap right before wp-settings.php loads, after the database constants. The last moment to act before WordPress builds its globals (e.g. swap in a profiling wpdb, which WordPress keeps when it already exists). Listen from a provider's register().
• Discovery scan sources — DiscoveryCacheManager::scans() lists, per location, whether the structures came from this process's copy (memory), the persistent cache (cache) or a real scan (disk), with time and count. The context's hit/miss counters only knew about the in-process copy, so a cold and a warm persistent cache both read as a miss.
• BindingResolved event — dispatched by BindingResolver for each binding resolved: source, field, attribute, post, time spent, whether this request's cache answered, whether a value came back. Only dispatched when a listener exists.
• AssetManager::containers() — every asset container by name (theme, plugins, modules), so a tool can list where a page's assets come from.

Tests

• FrontendControllerTest: condition + view recorded; index-fallback 404 recorded as not found; empty by default.
• WordPressBootingEventTest: event dispatched before the settings file (run as WP-CLI so no WordPress install is needed).
• DiscoveryCacheManagerSlowScanTest: disk vs memory; disk vs warm persistent cache.
• BindingResolverTest: resolutions announced, cache hits flagged, empty values flagged.
• AssetManagerContainersTest: containers listed by name.

Rector, Pint and PHPStan clean; type coverage 99.2 %. Locally 11 QueryException failures in Hook/Async and ModuleActivation exist on the base branch too (12 there), unrelated to this change.

Part of the Debugbar integration spec (changes 1, 2, 4, 6 and 7; pollora/debugbar launches with v13.35.3).