READING LIBRARY 4.9.3 QA

PASS: Node JavaScript syntax checking.
PASS: Chromium DOM load / no JavaScript page errors.
PASS: YouTube sidebar button opens dedicated page and highlights active nav.
PASS: Discover does not contain the YouTube banner or YouTube search.
PASS: Discover Free still finds 103 bundled free resources.
PASS: Discover and YouTube search inputs retain their independent values.
PASS: YouTube subject tabs update locally and search-channel URL is encoded.
PASS: All five browse-by-subject topic links exist.
PASS: Individual video rendering/search/subject filtering with a sample TEST ONLY ID.
PASS: Help & Support newsletter link matches supplied MailerLite share URL.
PASS: Newsletter links open in a new tab with noopener/noreferrer.
PASS: At 414 px viewport, layout has no page-wide horizontal overflow.
PASS: ZIP integrity, mandatory files and version tag.

LIMITS: External YouTube and MailerLite destinations could not be verified
from within the restricted browser test environment. Existing video map
has no video IDs; no individual video URLs were invented or shipped.
