fix(plugins): don't treat transient absence from /api/plugins as uninstall (#741)

* fix(plugins): don't treat transient absence from /api/plugins as uninstall

The backend clears its plugin registry at the start of load_plugins()
and repopulates it incrementally while HTTP stays up, so every backend
restart (desktop: Audio Quality soundfont switch, LAN toggle, update
restart) serves a window of partial — even empty — /api/plugins
responses. loadPlugins() treated absence from the current response as
an uninstall, with three destructive consequences for still-loaded
plugins:

1. Their settings-panel and screen DOM were wiped while their
   _loadedPluginScripts entry survived, so the NEXT refetch failed the
   DOM-existence check and re-evaluated the plugin's screen.js
   mid-session. For the desktop audio_engine plugin that re-ran init()
   against the surviving native audio chain and exactly duplicated
   every VST/NAM/IR stage (the alpha testers' "chain duplicates after
   leaving the Audio menu" / blown-out gain reports).
2. _reconcilePluginStyles dropped their stylesheet, leaving them
   visible but unstyled until they reappeared.
3. The stale-contribution sweep unmounted their UI contributions and
   unregistered their capability participant with no re-registration
   path (plugin scripts don't re-run thanks to the loadedScripts
   guard).

Absence is now a non-signal everywhere in loadPlugins: the DOM wipe and
style reconcile are scoped to plugins the response actually names, and
the absence sweep is removed. Present plugins still fully re-sync via
_registerLegacyPluginUiContributions each round; failed plugins are
present in the response and still cleaned up; nav is rebuilt from the
response so genuinely uninstalled plugins drop out of it, and their
(un-unloadable) already-evaluated scripts keep their DOM until reload.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* test: update idempotence contract to the absence-is-not-uninstall invariant

The removed-plugin sweep contract pinned the old behavior this branch
deletes; pin the new invariant instead (no absence sweep + respondedIds
scoping on the DOM/style reconcilers).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

tests/test_plugin_runtime_idempotence.py

@@ -38,15 +38,25 @@ def test_plugin_loader_unmounts_previous_ui_contributions_before_reregistering()
     assert "await _commandUiDomain(contribution.domain, 'mount', plugin, contribution)" in source
 
 
-def test_plugin_loader_unmounts_contributions_for_removed_plugins():
+def test_plugin_loader_does_not_treat_response_absence_as_uninstall():
+    # A plugin transiently absent from /api/plugins (the backend clears its
+    # registry at the start of load_plugins() and repopulates incrementally
+    # while HTTP stays up, so restarts serve partial responses) must NOT be
+    # torn down: the old absence sweep unmounted UI contributions and
+    # unregistered the capability participant with no re-registration path
+    # (plugin scripts don't re-run), and the DOM/style wipes forced a
+    # mid-session screen.js re-evaluation that duplicated the desktop
+    # audio_engine's native signal chain.
     source = (ROOT / "static" / "app.js").read_text(encoding="utf-8")
 
-    assert "const livePluginIds = new Set(plugins.map((plugin) => plugin.id))" in source
-    assert "for (const [pluginId, contributions] of _pluginUiContributions)" in source
-    assert "const stalePlugin = { id: pluginId }" in source
-    assert "await _commandUiDomain(contribution.domain, 'unmount', stalePlugin, contribution)" in source
-    assert "window.feedBack?.capabilities?.unregisterParticipant?.(pluginId)" in source
-    assert "_pluginUiContributions.delete(pluginId)" in source
+    # The absence-triggered sweep is gone (rationale comment in its place)...
+    assert "const livePluginIds" not in source
+    assert "const stalePlugin = { id: pluginId }" not in source
+    assert "deliberately NO stale-contribution sweep" in source
+    # ...and the DOM/style reconcilers only act on plugins the response names.
+    assert "const respondedIds = new Set(plugins.map((p) => p.id))" in source
+    assert "respondedIds.has(pid) && !alreadyHydrated.has(pid)" in source
+    assert "responded.has(id) && !styled.has(id)" in source