Fix WEBrick range requests for local video playback

WEBrick's not_modified? treats a Range request's If-Range header like
If-None-Match, replying 304 with no body instead of 206 with the
requested bytes. Browsers rely on this to fetch a video's trailing
moov atom, so jekyll serve was breaking playback for videos recorded
with metadata at the end of the file. Dev-server-only; production is
unaffected since it isn't served by WEBrick.

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

_source/_plugins/webrick_range_fix.rb

@@ -0,0 +1,16 @@
+# 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
+#
+require 'webrick'
+
+module WebrickRangeFix
+  def not_modified?(req, res, mtime, etag)
+    req['Range'] ? false : super
+  end
+end
+
+WEBrick::HTTPServlet::DefaultFileHandler.prepend(WebrickRangeFix)