[meta] Search API Fast 3.x: a modern rewrite

Summary

This is the tracking (meta) issue for Search API Fast 3.x, a
complete rewrite of the module on a modern, layered architecture targeting
Drupal 11 and PHP 8.3+. The proven 2.x
design — sharding the items of an index across many parallel Drush workers
backed by database queues — is preserved, but every part is re-implemented
with dependency injection, value objects, strict types and full test
coverage, and the feature set is extended with a UI dashboard and better CLI
feedback.

Work happens on the 3.0.x branch. Per-topic child issues (below)
let the community and maintainers test and give feedback on one area at a
time.

Why a 3.x rewrite?

• Security & robustness. 2.x spawned and managed workers
  with raw shell strings (exec(), nohup,
  ps, pkill), which is fragile and a command-
  injection surface. 3.x uses the Symfony Process component with argument
  arrays (no shell) and tracks processes by PID.
• Portability. 2.x effectively required Search API Solr and
  a specific environment. 3.x works with any Search API backend and targets
  current Drupal/PHP.
• Maintainability. A few large classes became a layered,
  dependency-injected design with unit, kernel and functional tests and CI
  quality gates.
• User experience. A working UI dashboard, and a Drush
  command that runs in the background by default with an opt-in, TTY-aware
  live progress bar (--progress), which also addresses reviewer
  feedback on the 2.x progress work.

What's different from 2.x

Roadmap

Done (on 3.0.x, CI green)

• Layered architecture, D11 / PHP 8.3, Symfony Process worker engine.
• Drush search-api-fast:index: background by default, --progress for a live bar, --exclude, and a search-api-fast:stop command.
• Fast indexing UI dashboard (per-index tab + status-form embed).
• Reliable stop and respawn-safe progress tracking.
• Settings form and configuration (incl. settings.php override).
• Unit / Kernel / Functional tests and CI quality gates.
• README, CHANGELOG and UPGRADING documentation.

In progress / needs community testing

• Broader real-world testing across backends, hosting and index sizes.
• Per-topic feedback via the child issues below.

Child issues

• Fast indexing from the UI (dashboard) — #3592781
• Drush indexing with visual progress feedback — #3592783
• Test coverage and CI quality gates — #3592785
• Documentation (README, CHANGELOG, UPGRADING) — #3592787
• Exclude specific indexes when running over all indexes — #3592802
• Foundation: layered architecture (Drupal 11 / PHP 8.3) — #3592803
• Harden worker process spawning and termination (security) — #3592804
• Reliable stop and accurate progress tracking (respawn-safe) — #3592805
• Upgrade path and backwards compatibility (2.x → 3.x) — #3592806

How to test

composer require drupal/search_api_fast:3.0.x-dev
drush en search_api_fast
# CLI (background by default; --progress to watch, sapi-fast-stop to stop):
drush sapi-fast
drush sapi-fast my_index reindex --progress
drush sapi-fast-stop
# UI: /admin/config/search/search-api/index/MY_INDEX/fast
# Settings: /admin/config/search/search-api-fast

Requirements: Drupal 11, PHP 8.3+, Drush, and the PHP proc_open()
function enabled (the ext-posix extension and the setsid
command are recommended; the status report shows what is available).