feat(layout): support listener, loader, and component modifiers - #307

Summary

• Preserve the original Portman-owned SupportEvents registration unchanged.
• Register an independent SupportMagewireEvents adapter immediately after core events.
• Share layout argument lookup and overlays through LayoutArgumentOverlay, so features can opt into per-placement values without duplicating merge logic.
• Support listener additions, handler replacements, and keyed false / null removals through magewire:listeners.
• Support loader additions, replacements, and keyed null removals through magewire:loader; false remains a valid loader value.
• Accept an array of PHP ModifierInterface objects through magewire:modifiers. The interface extends Magento's block ArgumentInterface for layout object resolution. Each modifier receives a ComponentModifierContext with the component and the generic MagewireArguments collection; the context can expose other build data later without changing the interface.
• Preserve Magento Phrase values in loader effects. PHP modifier messages remain visible to i18n:collect-phrases.
• Keep Magento layout and resolver knowledge outside Portman-owned files.

Architecture

LayoutArgumentOverlay reads resolver arguments, replaces scalar values, appends list entries, and merges named entries with consumer-selected removal values. Event-specific listener normalization and dispatch guards remain in SupportMagewireEvents. The loader feature uses the same overlay before translating and dehydrating its effect, without mutating the component property.

ComponentModifierRunner applies declared modifiers in array order after block arguments are assembled and before component assembly, mount, or hydration. Modifiers can use MagewireArguments::get() and merge() to change any argument or change the component directly. Conditions can inspect Magento services and the component's initialized state at that point; snapshot state has not yet been hydrated.

<argument name="magewire:modifiers" xsi:type="array">
    <item name="configure" xsi:type="object" shared="false">Vendor\Module\Modifier\Configure</item>
</argument>

Active layout handlers are contributed during the event adapter boot hook, before server calls and dehydration. Tombstoned dispatches are rejected by a pre-call guard, and the adapter later filters the initial browser listener effect during dehydration. If core events are disabled, the adapter skips itself.

Listener precedence remains component listeners, then On attributes, then the layout overlay. Tombstones are resolved before dynamic placeholders are expanded. Whole-map clearing and live post-mount reconciliation remain out of scope.

Verification

• PHPUnit unit suite: 40 tests, 98 assertions locally.
• Mago lint and formatting checks, PHP syntax, XML parsing, JavaScript syntax, and git diff --check pass locally.
• Magento i18n:collect-phrases finds both PHP modifier messages.
• The /magewire/playwright/events route and Playwright spec exercise XML listeners, the loader overlay, and the modifier object (including PHP-computed listeners and Phrase messages); GitHub CI runs them against Magento.