* Make plugin registry hash independent of plugin discovery order
calculate_plugin_hash() iterates self.plugins.items() in insertion
order, which is the plugin discovery order of the local process. Two
processes can hold the same registry state (same plugins, versions,
active flags) in a different order and compute different hashes,
ping-ponging the _PLUGIN_REGISTRY_HASH setting and triggering endless
registry reloads in check_reload().
Sort by slug before hashing so the hash represents the registry state
rather than the iteration order of any particular process. Add a
regression test that reverses the plugin dict and asserts the hash is
unchanged.
* Address review comments: explicit sort key, guard against vacuous test
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Co-authored-by: Nasawa <christopher@anigeek.com>
