11: Loader boot and load path (merge after #9 and #10)

[nikolaystrikhar]

Adds Loader::boot(), load_all(), the five gates in front of the require_once, and the should_load filter.

Stacked on #10.

add_action( 'plugins_loaded', function () {
    Config::set_hook_prefix( 'give' );
    Loader::register( [ /* ... */ ] );
    Loader::boot();
}, 0 ); // priority 0 — before the absorber's own hooks fire

This is the library. Everything before it registers configuration and resolves objects; this is the code that decides whether a bundled plugin actually loads, and the first PR whose failure mode is a white screen rather than a wrong return value.

A sub-plugin is skipped when it is disabled, its guard constant is already defined, its dependencies are unmet, its file is missing or unreadable, or should_load returns false. The filter runs last, so it is only asked about a sub-plugin that would otherwise have loaded — and returning true cannot force a load past the guard constant.

• The , 0 is not decoration. boot() wires itself at plugins_loaded priority 2, and WordPress accepts, then never fires, a callback added at a priority the running dispatch has already reached. A late boot() used to load nothing at all, silently; now it is reported and the load runs inline.
• is_already_loaded() runs before are_dependencies_met(). Dependencies are an arbitrary host callable, and asking it first would let the library blame dependencies for a plugin the user can plainly see running.
• A missing hook prefix reports and returns, rather than throwing a Config_Exception out of a core action and fatalling the site.
• is_file() and is_readable() guard the require_once. file_exists() is true for a directory and for an unreadable file, and both fatal.

Documented rather than fixed: the bundled file is included from inside a method, so a top-level $var in it is function-local instead of global. No wrapper can change that, so the README says so. A parse error in a sub-plugin still fatals, deliberately.