Managed PHPUnit bootstrap drops init callbacks added during deferred init replay

[chubes4]

Problem

The managed wordpress.phpunit bootstrap replays deferred init callbacks from a fixed snapshot. When a plugin's early init callback registers callbacks at later init priorities, those callbacks never execute.

Data Machine Events exposes the normal WordPress pattern:

add_action( 'init', [ $plugin, 'init' ], 0 );
// During init():
add_action( 'init', [ $plugin, 'register_blocks' ], 15 );
add_action( 'init', [ $plugin, 'register_taxonomies' ], 20 );

Under a normal WordPress hook dispatch, the priority-15 and priority-20 callbacks run in the same init action. Under WP Codebox managed PHPUnit, did_action( 'init' ) > 0 but the blocks and taxonomies are absent.

The relevant bootstrap path snapshots/defer-replays callbacks in packages/runtime-playground/src/phpunit-command-handlers.ts around the managed bootstrap's pg_run_deferred_wordpress_hook_callbacks(..., 'init') call.

Expected

Managed PHPUnit replay must preserve normal WP_Hook behavior for callbacks added at later priorities during the same action. It must still execute each deferred callback exactly once and must not rerun callbacks WordPress already dispatched.

Acceptance

• Add a fixture callback at init priority 0 that registers another callback at priority 15.
• Prove both callbacks run once in priority order during managed bootstrap.
• Preserve existing dependency/component plugins_loaded, abilities, and duplicate-bootstrap guarantees.
• Do not require consumers to preload classes or invoke plugin methods directly.

Discovered while fixing Extra-Chill/data-machine-events#546.