[Track] Resolve $event paths and refresh DOM-backed context per dispatch

Start by reading AbstractTrack and TrackContext, then trace the existing placeholder resolution and dispatch path. Verify the behavior against the acceptance criteria: generic $event paths, recursive payload resolution, and fresh context and payload data after DOM morphs without remounting.

Goal

Let Track consume any event declaratively and keep its payload correct after partial DOM updates.

<form
  data-component="TrackShopify"
  data-track:fetch-update-after='{
    "event": "content_search_results",
    "result_count": "$event.detail.response.headers.x-search-result-count"
  }'>
</form>

A response-updated TrackContext should also be read at the time of this event.

Current limitation

Track only resolves $detail.* placeholders for CustomEvent.detail. This special alias does not expose the complete event and gives the placeholder system knowledge of one event subtype.

AbstractTrack also caches its payload and inherited context for the full mount cycle. When Fetch morphs a JSON script or context attribute, an existing Track instance can publish stale context.

Proposed behavior

Generic event paths

Replace $detail.* with $event.*:

"$event.detail.response.headers.x-search-result-count"
"$event.detail.request.searchParams.tag"
"$event.target.dataset.type"

The resolver should:

• work for native events and CustomEvent;
• traverse plain objects and arrays;
• return undefined for a missing path;
• recurse through nested objects and arrays in the declared payload;
• remain independent from Fetch and other event producers.

This is an alpha release, so the obsolete $detail.* syntax can be removed instead of retained as a compatibility alias.

Fresh DOM-backed data

Resolve these sources at dispatch time:

• inherited TrackContext;
• the Track payload script;
• the Track payload option.

Do not retain a mount-cycle cache that survives a morph update. TrackContext already reads its own script and attributes through getters, so Track should use the current value when sending.

Acceptance criteria

• $event.detail.* works with CustomEvent and component lifecycle events.
• $event.target.* works with native events.
• Nested payload placeholders resolve recursively.
• A morphed TrackContext script affects the next dispatch without remounting Track.
• A morphed payload script affects the next dispatch.
• Existing context and event payload merge precedence remains unchanged.