feat(plugin-contract): prove exact unload after a reload #36

Feature description

The plugin-contract reload scenario (#34, #35) proves load, unload, load: the second load restores the first load's surface and its callback still works. It never unloads again after that second load, and the ordinary scenario never unloads a reloaded plugin. A plugin whose unload is exact the first time but leaks after a reload therefore passes every case.

Related Code

tests/_support/plugin-contract/scenario.zsh in #35 (head 8799b23), lines 68-83: the reload scenario ends after the effect check, and its comment leaves exact unload to ordinary.

Reproduced on 8799b23 with a fixture whose unload removes its precmd hook only once, keyed on a _zunit_* flag the observer excludes (the device reload-stale uses):

reload   noninteractive adv-second-unload: rc 0 (passes)
ordinary noninteractive adv-second-unload: rc 0 (passes)

The shipped fixture unloads exactly after a reload, so the check has a positive case today.

Additional Context

Found by the second independent review of #35 and left out of it by maintainer decision, so #35 merges as it stands with Refs #34. The organization testing standard asks that a separate clean-shell case load, unload and load again; it does not name the unload that follows, so this closes a gap the standard leaves open rather than one it requires.

Proposed change:

• In the reload scenario, snapshot before the first load, then after the effect check unload again and assert plugin_restored against that first snapshot.
• Add a negative fixture whose unload leaks only after a reload; it must fail reload and pass ordinary.
• Reword the scenario comment that leaves exact unload to ordinary.

Acceptance criteria:

• A fixture whose post-reload unload leaks fails the reload case, and the shipped fixture passes it.
• ./zunit --tap tests passes on the native job.

Self-service

• I'd be willing to address this myself.

Have you read the Contributing Guidelines?

• I have read the Contributing Guidelines.

Are you familiar with the Contributor Covenant Code of Conduct?

• I have read the Contributor Covenant Code of Conduct.