Prevent page caches from storing content-negotiation redirect

Full-page caches (LiteSpeed Cache, WP Super Cache, etc.) ignore
the Vary: Accept header and cache the 303 redirect keyed by URL
alone, causing all visitors to be redirected to the .md URL.

Add three cache-prevention layers before the redirect:
- DONOTCACHEPAGE constant (universal WP cache convention)
- Cache-Control: private, no-store (HTTP standard for proxies/CDNs)
- litespeed_control_set_nocache action (LSCWP API, no-op if inactive)

The .md URLs themselves remain fully cacheable.

Fixes #30

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: TBarregren

src/Router/RewriteHandler.php

@@ -372,6 +372,16 @@ public function handle_accept_negotiation(): void {
             return;
         }
 
+        // Prevent page caches from storing this content-negotiation redirect.
+        // LiteSpeed Cache (and similar full-page caches) ignore the standard
+        // HTTP Vary header and would serve this 303 to every visitor regardless
+        // of their Accept header value.
+        if (!defined('DONOTCACHEPAGE')) {
+            define('DONOTCACHEPAGE', true);
+        }
+        header('Cache-Control: private, no-store');
+        do_action('litespeed_control_set_nocache', 'Content negotiation redirect');
+
         // 303 See Other: redirect to markdown URL with Vary for cache correctness.
         header('Vary: Accept');
         wp_safe_redirect($md_url, 303);