[botandbang]

What and why

Fixes LOC-443.

Every first-party plugin's tests ran its own host over pipes, eleven copies of the same handshake, threads and cancel plumbing. They now use FakeHost, which the interop plugin already moved to. A plugin author outside the repo gets the same harness the first-party plugins use.

Added to FakeHost (behind test-util, no protocol change):

• on_call(method, respond), where respond returns a Response: Answer, AnswerThenCancel (answer the call, then cancel the hook, as a host does when the user stops it mid-call) or Hold (never answer, until the hook is cancelled). on_read now goes through it.
• Sent::wait_timeout(limit), for tests that check a hook stops.
• send keeps a context.scratch the params already give, including null for a host without blobs, instead of always making a folder.

Removed: test_util::Canceller, which only these harnesses used.

Changed, in the plugins' tests (demos, tracklist, sound, coverart, musicbrainz, archive, plyr, soundcloud, mixcloud, youtube, interop's Traktor reader):

• Every test keeps its name, its assertions and its Hegel generators and case counts. Only the host under it changed.
• Each plugin's scripted service (the archive, plyr, SoundCloud, Mixcloud and YouTube upload scripts, coverart's routes, musicbrainz's busy replies) answers through on_call.
• The publishing plugins' harnesses check on every case that the plugin called for nothing but net/fetch, as their per-call asserts did.
• sound's and coverart's event tests hold the last event's call, so the host doesn't stop while an event handler is still reading. FakeHost's docs describe that pattern.
• The explicit describe checks are gone: FakeHost::start already checks describe answers the manifest.

The README's "Testing a plugin" section and apps/locallibrary/CONTRIBUTING.md describe the new calls.

How it was tested

• cargo nextest run on loclib-plugin and the 11 plugin crates: 476 passed.
• New FakeHost tests: an_answer_then_cancel_reaches_the_plugin_first, a_held_call_waits_until_the_hook_is_cancelled, a_scratch_folder_given_is_kept, and the Hegel property prop_an_answer_reaches_the_plugin_unless_held. Each fails when its part is broken: cancelling before answering, answering a held call, wait_timeout never returning, or ignoring a given scratch folder.
• cargo clippy --all-targets -- -D warnings on the same crates, --all-features on loclib-plugin, cargo fmt --check, and the doctests.
• Coverage of the 12 crates (cargo llvm-cov, CI's ignore regex): 99.44% regions, 99.23% functions, 99.55% lines.

Checklist

• Tests cover the change
• The READMEs, docs/ and --help still say what the code does (no changelog line: tests only)

[linear-code]

LOC-443