Why

Administrator UX for com_content currently depends on full page submits for most list and edit interactions. This PR adds progressive asynchronous behavior with strict fallback compatibility, while preserving server-side ACL/CSRF enforcement and existing synchronous flows.

What this PR delivers

1) Async admin contract foundations

• Feature-flag plumbing for async backend behavior in com_content.
• Reusable JSON envelope trait for async responses.
• Shared client helper to execute async requests with graceful fallback.

2) Async list interactions in Articles manager

• Async state actions (publish, unpublish, archive, trash, featured, unfeatured).
• Async workflow transition execution.
• Async search/filter/order/pagination behavior with list fragment refresh.

3) Autosave in Article edit

• Autosave scheduler with dirty-state tracking.
• Async autosave endpoint contract and status payload.
• Versioning-safe autosave policy:
  • skip unchanged payload,
  • minimum interval throttle based on configured autosave interval.
• Recover last autosaved snapshot and undo recover (MVP, session-local).

4) Custom-field filtering in Articles list

• New custom-field filter controls in search tools.
• Query integration for selected custom field + optional value match.

5) Shared hooks + accessibility hardening

• Core reusable async fragment refresh helper extraction.
• Focus restoration after async refresh.
• Aria-live announcement for async messages.

6) Documentation and reviewer support

• Execution tracker with all units marked complete.
• Contract aligned with final implemented scope.
• QA and reviewer guide.
• PR handoff note.

Behavioral guarantees preserved

• Async behavior is feature-flag-gated.
• Non-async paths remain available and functional.
• CSRF and ACL checks remain server-side.
• Client fallback path is retained for unusable async responses.

Reviewer QA checklist (manual)

1. Flags OFF:
   • Confirm classic full submit behavior on list and edit screens.
2. Flags ON:
   • Confirm list async actions, transitions, search filters, pagination.
3. Autosave:
   • Confirm pending/saving/saved statuses.
   • Confirm skip-on-unchanged and skip-on-throttle behavior.
   • Confirm recover and undo recover controls.
4. Custom fields:
   • Confirm filter controls appear when fields exist.
   • Confirm list query narrows by selected field/value.
5. Accessibility:
   • Confirm focus restoration after async refresh.
   • Confirm async messages are announced via aria-live.

Risk and rollout notes

• Scope intentionally limited to com_content pilot for functional safety.
• Shared helper extraction is additive and consumed by pilot implementation.
• Follow-up work for additional components should reuse the same contract/hooks.