fix(scan): recover simple <video> detection site-agnostically

The no-mention-of-pornhub branch stopped finding plain <video> elements on
pages like thisvid.com. Two site-agnostic causes, both fixed without any
site-specific references:

1. Predicate over-rejection: isValidManifestUrl (and the nested isManifestUrl
   in m3u8DeepProbeFn) ran the placeholder/`%`/trailing-slash rejections
   BEFORE the concrete media signals. thisvid serves its direct MP4 WITH a
   trailing slash (.../14644093.mp4/?rnd=...), so `path.endsWith('/')`
   rejected a real, fetchable source. Concrete media signals (.mp4/.webm/.ogv/
   .mpd/.m4s/.m3u8/.hlsvod//hls/) now win outright, so a coincidental '%' or
   trailing slash never sinks a real source — while genuine route templates
   (/playlists/, /playlist/%1%/, /videos/{id}/) are still rejected.

2. Runtime cause on ad-iframe-heavy pages: scrapeTab ran only a MAIN-world
   scrape. A MAIN-world + allFrames injection can reject on pages with
   sandboxed/opaque-origin ad iframes, returning 0 videos even when a <video>
   is visible. scrapeTab now falls back to an isolated-world scrape (no `world`
   key — always permitted) when the MAIN-world run finds nothing. The per-frame
   reduction was extracted into a shared reduceScrapeFrames helper.

Adds __tests__/simple_video.test.mjs: a thisvid isValidManifestUrl battery
(trailing-slash MP4 accepted, templates still rejected, %2B/%2F tokens
preserved) plus a structural guard that scrapeTab keeps both a MAIN-world
primary and an isolated-world fallback.

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

__tests__/run.mjs

@@ -7,6 +7,7 @@ import './data_flow.test.mjs';
 import './viewer_integration.test.mjs';
 import './config_validation.test.mjs';
 import './hls_qa.test.mjs';
+import './simple_video.test.mjs';
 import './referer_proxy.test.mjs';
 import { run } from './runner.mjs';