feat(runtime): stage VM runtime artifacts and writable drives in per-VM ephemeral storage by default

readme.md

@@ -257,9 +257,22 @@ const clonePath = await imageManager.cloneRootfs(rootfsPath, 'ubuntu-clone.ext4'
   bin/<version>/jailer
   kernels/<name>
   rootfs/<name>
-  sockets/<vmId>.sock
+
+/dev/shm/.smartvm/runtime/<vmId>/
+  firecracker.sock
+  drives/<driveId>-<sourceName>
 ```
 
+Runtime files default to `/dev/shm/.smartvm/runtime` on Linux when available, so sockets and per-VM writable drive copies live on tmpfs and are removed during cleanup. Set `runtimeDir` to override this.
+
+**Ephemeral drives:**
+
+- Writable drives are copied into the per-VM runtime directory before boot by default
+- Firecracker receives the staged tmpfs path, so guest writes do not modify cached/base rootfs files
+- Per-VM staged drive files are deleted by `vm.cleanup()` / `smartvm.cleanup()`
+- Read-only drives are not copied unless `ephemeral: true` is set on the drive
+- Set `ephemeral: false` on a drive, or `ephemeralWritableDrives: false` on `SmartVM`, only when persistence is explicitly required
+
 ### `BaseImageManager` — Base Images
 
 Downloads known base image bundles into a `/tmp` cache for integration tests and quick local smoke tests. The default preset is `latest`; `lts` maps to a pinned Firecracker CI train (`v1.7`) for a stable fallback. Hosted project-owned manifests are also supported for pinned Alpine/BusyBox-style bundles.
@@ -304,6 +317,7 @@ Example configuration:
 ```typescript
 const smartvm = new SmartVM({
   baseImageCacheDir: '/tmp/.smartvm/base-images',
+  runtimeDir: '/dev/shm/.smartvm/runtime',
   maxStoredBaseImages: 4,
   baseImageManifestUrl: 'https://assets.example.com/push.rocks/smartvm/base-images/smartvm-minimal-v1/x86_64/manifest.json',
 });
@@ -542,6 +556,7 @@ All configuration interfaces are fully exported for type-safe usage:
 ```typescript
 import type {
   ISmartVMOptions,
+  IMicroVMRuntimeOptions,
   IMicroVMConfig,
   IBootSource,
   IMachineConfig,