Prevent page caches from storing content-negotiation redirect

Many full-page caches do not vary their cache key by the Accept
request header. When they store the 303 redirect from content
negotiation, all subsequent visitors are redirected to the .md
URL instead of seeing the HTML page.

Two complementary layers prevent this:

- Cache-Control: private header prevents shared caches (CDNs,
  reverse proxies) from storing the redirect while allowing
  browser caching.

- DONOTCACHEPAGE constant and LiteSpeed API call tell WP-level
  page caches not to store the response. This is behind a filter
  (markdown_alternate_disable_page_cache_on_redirect) so sites
  with Vary-aware caching can opt out.

The .md URLs themselves remain fully cacheable.

Fixes #30

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

Author: TBarregren

src/Router/RewriteHandler.php

@@ -372,6 +372,22 @@ public function handle_accept_negotiation(): void {
             return;
         }
 
+        // Prevent shared caches (CDNs, reverse proxies) that don't honour
+        // Vary from storing this content-negotiation redirect and serving
+        // the 303 to every visitor regardless of their Accept header.
+        header('Cache-Control: private');
+
+        // Many WordPress page-cache plugins cache at the PHP level and may
+        // not vary by the Accept request header. Tell them explicitly not to
+        // cache this response. Sites with Vary-aware caching can disable this:
+        //   add_filter( 'markdown_alternate_disable_page_cache_on_redirect', '__return_false' );
+        if ( apply_filters( 'markdown_alternate_disable_page_cache_on_redirect', true ) ) {
+            if ( ! defined( 'DONOTCACHEPAGE' ) ) {
+                define( 'DONOTCACHEPAGE', true );
+            }
+            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);