Link upstream WEBrick PRs in range-fix comment

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

_source/_plugins/webrick_range_fix.rb

@@ -0,0 +1,20 @@
+# Dev-server-only fix: WEBrick's FileHandler#not_modified? treats a Range
+# request's If-Range header the same as If-None-Match, replying 304 (no body)
+# instead of 206 with the requested bytes. Browsers rely on ranged requests
+# (with If-Range) to fetch a video's trailing moov atom when seeking/loading
+# metadata, so this breaks video playback under `jekyll serve`.
+# See https://github.com/ruby/webrick/blob/master/lib/webrick/httpservlet/filehandler.rb
+#
+# Upstream bug, not yet fixed as of writing:
+# https://github.com/ruby/webrick/pull/173 (fix if-range returning 304 instead of 206/200)
+# https://github.com/ruby/webrick/pull/169 (do not cache range requests)
+# Drop this plugin once one of those lands in a released webrick version.
+require 'webrick'
+
+module WebrickRangeFix
+  def not_modified?(req, res, mtime, etag)
+    req['Range'] ? false : super
+  end
+end
+
+WEBrick::HTTPServlet::DefaultFileHandler.prepend(WebrickRangeFix)