Always use Firecracker Diff snapshots (fast, only changed pages) and
merge diff files at the file level when the generation cap is reached.
The previous approach used Firecracker's Full snapshot type which dumps
all memory to disk and can timeout, losing all snapshot data on failure.
Add snapshot.MergeDiffs() which reads each block from the appropriate
generation's diff file via the header mapping and writes them into a
single consolidated file with a fresh generation-0 header.
Use Firecracker's Diff snapshot type when re-pausing a previously
resumed sandbox, capturing only dirty pages instead of a full memory
dump. Chains up to 10 incremental generations before collapsing back
to a Full snapshot. Multi-generation diff files (memfile.{buildID})
are supported alongside the legacy single-file format in resume,
template creation, and snapshot existence checks.
Implement full snapshot lifecycle: pause (snapshot + free resources),
resume (UFFD-based lazy restore), and named snapshot templates that
can spawn new sandboxes from frozen VM state.
Key changes:
- Snapshot header system with generational diff mapping (inspired by e2b)
- UFFD server for lazy page fault handling during snapshot restore
- Stable rootfs symlink path (/tmp/fc-vm/) for snapshot compatibility
- Templates DB table and CRUD API endpoints (POST/GET/DELETE /v1/snapshots)
- CreateSnapshot/DeleteSnapshot RPCs in hostagent proto
- Reconciler excludes paused sandboxes (expected absent from host agent)
- Snapshot templates lock vcpus/memory to baked-in values
- Proper cleanup of uffd sockets and pause snapshot files on destroy
