Keep panel loaders across plugin changes so the OSD loads once (#13439)

Every plugin change handed the panel Instantiator a fresh array, which
rebuilds every panel. The panels were rebuilt four times in the first
700ms of startup, each rebuild starting a new asynchronous load of every
keepLoaded panel while the last was still in flight. Under load two OSDs
could run at once, each registering the osd IPC handler: the runtime
smoke test's intermittent "another handler is registered for target
osd".

The entries now live in a ListModel synced in place. A panel whose
plugin still loads the same way keeps its loader, so startup adds each
panel once, and a full plugin reload still clears the model to rebuild
from fresh code.

Under a six-core busy load the smoke test hit the duplicate in 1 of 5
runs on quattro and in none of 10 with this change.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

test/shell.d/panel-entries-test.sh

@@ -0,0 +1,30 @@
+#!/bin/bash
+
+set -euo pipefail
+
+source "$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd)/base-test.sh"
+
+run_node_test <<'JS'
+const fs = require('fs')
+const shellQml = fs.readFileSync(path.join(root, 'shell/shell.qml'), 'utf8')
+
+// Handing the Instantiator a fresh array rebuilt every panel on each plugin
+// change, several times during startup, so a keepLoaded panel such as the OSD
+// could load twice at once and register its IPC handler twice.
+assert(
+  /Instantiator \{\s*model: panelEntryModel/.test(shellQml) && !shellQml.includes('panelEntries'),
+  'panel loaders come from a model synced in place, not a reassigned array'
+)
+assert(
+  /if \(next && next\.kind === row\.entryKind && next\.keepLoaded === row\.keepLoaded && next\.sourceUrl === row\.sourceUrl\) \{\s*delete wanted\[row\.pluginId\]/.test(shellQml),
+  'a panel whose plugin still loads the same way keeps its loader'
+)
+assert(
+  /function unloadPanels\(\) \{[\s\S]*?panelEntryModel\.clear\(\)/.test(shellQml),
+  'a full plugin reload still rebuilds every panel from fresh code'
+)
+assert(
+  (shellQml.match(/shell\.syncPanelEntries\(\)/g) || []).length === 2,
+  'plugin changes and finished scans sync the panel entries'
+)
+JS