Add instrumented extraction fixture tests for WHT/Ars Technica

BACKLOG's "Extraction test coverage for WHT / Ars Technica" item:
ReaderExtractor's site-specific preprocessing had only ever been
diagnosed by hand against single live articles across several rounds
of fixes. Four hand-authored fixtures now pin down the exact behavior:

- West Hawaii Today's paywall-div unhide (class="paywall paywall_*")
- Lazy-loaded image src resolution (data-src -> src)
- Ars Technica's duplicate PhotoSwipe caption removal
  (.pswp-caption-content vs. the real <figcaption>)
- Ars's split .post-content.post-content-double block merge

Needs a real WebView to run Readability.js, so these are instrumented
tests (app/src/androidTest/), not plain JVM unit tests -- that source
set didn't exist yet (testInstrumentationRunner + androidx.test deps
added to app/build.gradle.kts). Fixtures are minimal hand-authored
markup targeting the exact selectors ReaderExtractor's own code uses,
not scraped live pages, which rot and would fail for reasons unrelated
to the extraction logic.

Fixtures are served over real http:// via MockWebServer, since
ReaderExtractor's WebView has allowFileAccess = false and won't load
file:// URLs. Added a debug-only network security config override
(app/src/debug/res/xml/network_security_config.xml) permitting
cleartext to 127.0.0.1/localhost for this -- release is untouched.

Verified the tests actually catch regressions, not just pass
trivially: temporarily broke the paywall-unhide selector, confirmed
the corresponding test failed on both connected emulators, then
reverted and confirmed green again.

Ran on 2 connected devices (feedreader_test, doot_test_api36): 4/4
passing, 0 failures. Instrumented tests aren't counted by the JVM-only
jacoco coverage metric (expected, not a gate miss -- see CLAUDE.md).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017GMEkHeqKz6FLkmizowBTK

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