Actually resolve shared links, and guard against dead code

Bug report from the phone: pasting a link produced "Could not parse feed:
Unexpected <! (position:START_DOCUMENT...)" -- the XML parser meeting
<!DOCTYPE html>. The cause was mine. SubscriptionResolver.findFeedLink and
applePodcastId were written, unit-tested, documented as how the share sheet
would work, and had zero call sites. The raw URL went straight to the parser.

FeedResolver now does what I said the app already did: a direct feed resolves
to itself, a show's web page resolves through its link rel=alternate, an Apple
Podcasts link goes through the iTunes lookup, and a YouTube channel or
playlist uses YouTube's own RSS endpoint. Both entry points -- the in-app add
box and the share sheet -- go through it, so they cannot drift apart again.

Verified on-device against real sites: thisamericanlife.org resolved to
/podcast/rss.xml, a direct megaphone feed passed through unchanged, and a
YouTube handle failed with an explanation rather than opaquely.

Errors are now written for a person. "Unexpected <!" told the user nothing;
"That's a web page, not a podcast feed, and it doesn't link to one" tells them
what to do. A test asserts parser internals never reach the UI.

Fixing the class, not just the instance:

- A reachability test fails when a helper object's function has no production
  caller. Its first version passed on the very code it was written to catch,
  because a bare name also matches a KDoc cross-reference -- it now requires a
  call-shaped match. Verified by reintroducing the original bug and watching
  it fail.
- That audit found a real functional gap, not just unused code:
  QueueRepository.advance had no caller, so an episode ending never advanced
  the queue. Playback now marks the episode played, advances, and starts the
  next one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018AwnALZL6bD4VBtATDVv3B

1 files changed, 1 insertions, 1 deletions

diff --git a/.coverage-baseline b/.coverage-baseline
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-70.75
+71.77