M2: storage layer (all-files + SAF backends, volumes, access policy) + 32 JVM tests

CHANGELOG.md

@@ -12,6 +12,17 @@ upload needs a **higher `versionCode`** than every previous upload.
 
 ### Added
 
+- **M2 — storage layer.** The two storage backends behind one interface. `AllFilesSource`
+  is `java.io.File` over every mounted volume (the "All files access" path);
+  `SafSource` is the Storage Access Framework fallback, reading a persisted tree URI
+  through `DocumentsContract` with `SafDocumentIds` doing the tree↔path mapping and
+  `SafGrant` owning the folder picker and its persistence. `VolumeRepository` enumerates
+  volumes via `StorageManager`/`StorageVolume` and reports free/total space through
+  `StatFs`; `StorageAccess` and `StorageAccessPolicy` decide between the two backends.
+  Both sources confine their blocking I/O to `Dispatchers.IO` and expose a `suspend`
+  contract. 32 new JVM tests (76 total), including a Robolectric run of `SafSource`
+  against a stand-in `DocumentsProvider`.
+
 - **M1 — pure-Kotlin core engine.** An Android-free `core/` package holding the logic
   every other layer builds on: `StoragePath` (normalised absolute paths that can never
   escape the root), `FileEntry` with extension-based `FileKind` classification and a