Make the plugin file the composition root

[GaryJones]

The Plugin class had become a single-responsibility class wearing the wrong name: everything it did was the settings page. ARCHITECTURE.md is unambiguous that a plugin does not need a Plugin class at all — the main file defines one constant, loads the autoloader, and wires its features through a bootstrapper. My earlier brightnucleus change kept a Plugin class and injected __FILE__ into it, which quietly diverged from that. This realigns it.

plugin-slug.php is now the composition root: it defines PLUGIN_SLUG_FILE — the single constant every path derives from at runtime — and on plugins_loaded hands off to a Bootstrapper. The bootstrapper is the one place features are registered; today it registers a SettingsPage, which holds what Plugin used to, and new features slot in beside it. No Plugin class, no globals.

The one wrinkle is that referencing PLUGIN_SLUG_FILE inside a class is what previously blocked PHPStan's top level, since PHPStan does not evaluate the runtime define(). Rather than route around it by injecting the path, a small bootstrap file declares the constant for analysis, so the classes use it directly — as the standard writes it — and analysis stays at max.

The boilerplate stays flat and container-less; the DI container and layered namespaces ARCHITECTURE.md shows are the scale-up path, not something to impose on a single settings page. The tests split to match the new classes, the bootstrapper's wiring is covered with a @uses annotation for the feature it registers, and mutation coverage holds at 100%.