doc: dependencies when moving PuterFS to extension

This commit begins the plan towards moving PuterFS to an extension. It
is first necessary to analyze the dependencies and determine which
services and modules are part of PuterFS and which ones are not.

doc/planning/2025-10-21_puter-fs-extension.md

@@ -0,0 +1,79 @@
+## 2025-10-21
+
+### Moving PuterFSProvider to an Extension
+
+PuterFSProvider is not trivial to move to an extension because of
+relative imports (`require()`s) which represent dependencies on parts
+of Puter's core which may not be available to extensions, or should
+move with PuterFSProvider into an extension.
+
+Dependencies of PuterFS provider will be placed into the following
+categories:
+- **Already OK** - this is already exposed to extensions
+- **Export As-Is** - this needs to be exposed to extensions
+- **Belongs to PuterFS** - this needs to be moved to an
+  extension first or at the same time as PuterFSProvider
+- **Create Extension API** - an API needs to be created or improved
+  to use this dependency in the corrrect way for PuterFSProvider to
+  be an extension
+  
+External dependencies (such as `uuid`) and dependencies treated like
+external dependencies (such as `putility`) are not included here
+because they're just updates to a `package.json` file.
+
+#### Already OK
+- Context
+- APIError
+- `DB_WRITE`, `DB_READ`
+- streamutil
+- config
+- Actor
+- UserActorType
+- get_user
+- metering service
+- trace service
+
+#### Export As-Is
+- ~~filesystem selectors~~
+- fsCapabilities
+- UploadProgressTracker (utility)
+- FSNodeContext
+- ResourceService constants
+- ParallelTasks
+- FSNodeContext type context (`TYPE_FILE`, etc)
+- operation frame status constants
+
+#### Belongs to PuterFS
+- FSLockService
+- FSEntryFetcher
+- FSEntryService
+- `update_child_paths` [^1]
+- SizeService
+- `storage` object from **Context** [^2]
+
+[^1]: FilesystemService belong's in Puter Core, but
+      the `update_child_paths` method is an
+      implementation detail of PuterFS
+[^2]: LocalDiskStorageService registers this value
+      in the `context` using the `context-init` service.
+      PuterFS as an extension should emit an event where
+      other extensions can register a PuterFS storage
+      strategy.
+
+#### Create Extension API
+
+See notes below for details
+- filesystem selectors
+- access current operation frame
+- getting/creating actors from user ID
+
+### New Extension APIs
+
+#### Filesystem Selectors
+
+Filesystem selectors can be implied from strings instead
+of having to instantiate classes and compose them.
+
+Path: `"/just/a/string"`
+UUID: `/^[^\/\.]/`
+Child: `SOME-UUID/followed/by/a/path`