Plug-ins: settings and the cache total stay in memory between storage calls

A settings get no longer reads and parses settings.json each time. The
parsed settings are kept per plug-in folder and read again only when the
file's size or modified time changes. A set to the value a key already
has writes nothing, and every other change is written with an atomic
write before the call returns. plugin.remove drops the kept copy.

cacheBytes in plugin.list, plugin.info and plugin.install comes from a
total kept while the cache folder keeps its modified time, instead of a
listing of up to 8,192 items per call. A cache write still lists the
folder to enforce the caps.

docs/plugins.md states what a run counts and that a cache call moves a
whole entry in one step. docs/plugins.md and docs/mcp.md now say when a
later plugin.remove retries a leftover .removed folder.

docs/plugins.md

@@ -269,10 +269,16 @@ the key as `(key, key_len)`, the UTF-8 bytes at `key` in its memory:
 A call that fails returns `-1` when the key has no value, `-2` when a key, value or buffer doesn't lie inside the
 module's memory, `-3` when the key is not a valid key, `-4` when the write would pass a cap (nothing changes), `-5`
 when there is nowhere to keep anything, `-6` when reading or writing the file failed, and `-7` when the run has used
-its 10,000 storage calls or its 1 GiB of storage reads and writes. Each call counts the bytes it reads and writes: a
-settings call the whole settings file it reads, and a set or delete also the whole file it writes; a cache call the
-entry it reads or writes. A read counts them even when the value doesn't fit and nothing is copied. A call that would
-pass the 1 GiB returns `-7` before it reads or writes anything.
+its 10,000 storage calls or its 1 GiB of storage reads and writes. Each call counts the bytes it reads and writes. A
+settings call counts the whole settings file when it reads the file, and a set or delete also counts the whole file
+when it writes the file; a cache call counts the entry it reads or writes. A read counts them even when the value
+doesn't fit and nothing is copied. A call that would pass the 1 GiB returns `-7` before it reads or writes anything.
+
+The app keeps the settings in memory after the first call and reads `settings.json` again only when the file's size
+or modified time differs from the last read or write. A get answered from memory reads no file and counts no bytes. A
+set to the value the key already has writes nothing. Every other set or delete writes the file before the call
+returns. A cache call reads or writes the whole entry in memory in one step, up to 64 MiB, and the run waits for it.
+A live effect runs while the art draws, so keep the entries it uses small.
 
 - A key is 1 to 64 characters from `A-Z`, `a-z`, `0-9`, `_` and `-`, and is not a Windows device name (`CON`, `PRN`,
   `AUX`, `NUL`, `COM0` to `COM9`, `LPT0` to `LPT9`, in any case). Keys differ in case: `tile` and `Tile` are two.
@@ -291,7 +297,9 @@ pass the 1 GiB returns `-7` before it reads or writes anything.
   (`plugin.install` with `replace: true`, after the user agreed) keeps the storage; `plugin.remove` deletes it, and a run of the plug-in still going gets `-5` from every storage call after that.
   `plugin.remove` first renames the folder (adding `.removed` to its name). When that fails, `plugin.remove` fails
   and the plug-in stays installed with its storage as it was. Files that can't be deleted after the rename stay in the
-  `.removed` folder, where no plug-in reaches them, and the next removal of a plug-in with that id deletes them.
+  `.removed` folder, where no plug-in reaches them. A later `plugin.remove` tries again to delete them when a plug-in
+  with that id is installed or a storage folder for the id exists, and that deletion can fail too. For an id that
+  isn't installed and has only a `.removed` folder, `plugin.remove` does not delete the folder and fails as not found.
   `plugin.remove` with the id of a plug-in that isn't installed deletes the storage kept for that id, when there is
   any.
   `plugin.clearCache` empties the cache and leaves the settings, and `plugin.list` and