Performance: Add static caching to FlattenedSiteSettingsLoader::loadAll() to prevent repeated cache lookups

### Problem/Motivation

The `FlattenedSiteSettingsLoader::loadAll()` method is called via `site_settings_preprocess()` for **every Twig
template** rendered on a page. Since the loader plugin is instantiated fresh each time via
`getActiveLoaderPlugin()`, the instance property `$this->settings` is always NULL, causing a cache backend lookup
on every single call.

On a typical node edit page with ~4,000 templates, this results in **~4,000 unnecessary cache lookups** per
request, significantly impacting performance.

### Performance Impact

Testing on a Drupal 10.5.6 node edit page:

| Metric | Without Fix | With Fix |
|--------|-------------|----------|
| `loadAll()` cache lookups | 4,085 | 1 |
| `MemcachedDriver::getMulti` calls | 4,933 | ~850 |
| Wall time for site_settings | 250ms | <1ms |
| Total page load | 3.1s | ~2.6s |

### Root Cause

In `site_settings.module`, the preprocess hook creates a new loader instance for each template, and since
`$this->settings` is an instance property (not static), each new instance makes a fresh cache lookup.

### Proposed Resolution

Add a static cache property that persists across plugin instances within a single request:

1. Add `protected static $staticSettingsCache = [];`
2. Check static cache before backend cache in `loadAll()`
3. Update `clearCache()` and `rebuildCache()` to maintain consistency

### Why More Visible in Drupal 10.5.6+

Drupal core changes (Issues #3548313, #3555720) between 10.5.4 and 10.5.6 changed entity revision queries,
resulting in ~3.4x more template renders on the same page, making this pre-existing inefficiency much more
noticeable.

### Related Issues

Similar fix for Metatag module: https://www.drupal.org/project/metatag/issues/357736