plugin.install refuses grant from a request unless the session was started
to allow it, so the test from main that installs with grant turns that on.
The install dialog's comment names OK's Sense::click.
`plugin.clearCache` without an id empties the cache of every plug-in
installed in the process. In the engine's library tests it could empty
the cache of another storage test running at the same time, and its
`bytesFreed` could count that test's cache. It now runs in
`crates/engine/tests/plugin_clear_cache.rs`, and the library test checks
that clearing one plug-in's cache leaves the other plug-in's cache.
The storage fuzz test starts each case with an empty plug-in store,
deletes the store when the case ends, and installs `fuzz.storage` with
`replace: true`, so the cases that shrink a failure install their
module.
docs/mcp.md says that files `plugin.remove` can't delete stay in the
folder renamed with `.removed` until the plug-in is removed again.
dotCooCoo