build(cloudflare): add a Cloudflare preview build target

DEPLOY_TARGET now selects the adapter; Netlify stays the default and NODE_ADAPTER
keeps working. Cloudflare compresses in transit, so the Worker build aliases
$lib/server/compression to a pass-through that leaves the streaming body alone.

CI builds and dry-run deploys this target. That is compilation and packaging
coverage only, not a production compatibility gate: native sharp still blocks the
share-preview route in the Worker runtime.

adapter.mjs

@@ -2,20 +2,11 @@ import process from 'node:process';
 import AdapterNode from '@sveltejs/adapter-node';
 import AdapterNetlify from '@sveltejs/adapter-netlify';
 
-export const nodeAdapter = process.env.NODE_ADAPTER === 'true';
-
-// Netlify is the deployment target; the node adapter exists so the service worker
-// build tests can check the `build/client` layout a Node server produces, and so Lighthouse CI
-// can audit a production build. Netlify's CDN compresses responses, so precompress here to match.
+export const nodeAdapter =
+	process.env.NODE_ADAPTER === 'true' || process.env.DEPLOY_TARGET === 'node';
+export const cloudflareAdapter = process.env.DEPLOY_TARGET === 'cloudflare';
 export const adapter = nodeAdapter
 	? AdapterNode({ precompress: true })
-	: AdapterNetlify({
-			// if true, will create a Netlify Edge Function rather
-			// than using standard Node-based functions
-			edge: false,
-
-			// if true, will split your app into multiple functions
-			// instead of creating a single one for the entire app.
-			// if `edge` is true, this option cannot be used
-			split: false
-		});
+	: cloudflareAdapter
+		? (await import('@sveltejs/adapter-cloudflare')).default()
+		: AdapterNetlify({ edge: false, split: false });