[6.x] Hybrid Glide image caching

[duncanmcclean]

This pull request implements a new "hybrid" caching strategy for Glide image manipulation.

The Problem

Statamic currently offers two Glide caching modes:

• Dynamic (cache: false) — Fast template rendering, but every image request hits PHP, even after the image has already been generated.
• Static (cache: true) — Images are served directly by the web server, but they're all eagerly generated during template rendering. A page with 10 images × 11 variants = 110 images blocking the page from even loading.

Hybrid Caching

Setting cache to 'hybrid' enables a new mode that combines the best of both:

// config/statamic/assets.php
'image_manipulation' => [
    'cache' => 'hybrid',
    'cache_path' => public_path('img'),
],

The Glide tag outputs a URL pointing to where the cached image will live, but doesn't generate the image during template rendering. When the browser requests the image:

1. If the file already exists on disk, the web server (Nginx/Apache) serves it directly — no PHP needed.
2. If the file doesn't exist yet, the request falls through to PHP, which generates the image, saves it to the public cache_path, and serves it.

After the first request, the web server serves the static file directly on all subsequent requests. The default Nginx/Apache/IIS configurations handle this automatically — no additional rewrite rules are needed.

How It Works

• A new GlideCachePathResolver predicts the deterministic cache path for an image without generating it.
• A new HybridUrlBuilder builds clean URLs using the cache path, and stores a mapping in the Glide cache store so the controller knows how to generate the image later.
• The GlideController handles the PHP fallback: it checks if the file already exists (logging a debug message if server rewrite rules aren't configured), looks up the mapping, and generates the image on demand.
• Cache invalidation and php please glide:clear work automatically — the cache disk is the public path, so existing cleanup logic handles it.

---

Closes statamic/ideas#1420

Docs PR: statamic/docs#1899

[daun]

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