Validate native CoW reflinks on Btrfs via loopback device

[Rakshat28]

Btrfs is a primary target filesystem for bdstorage because of its native Copy-on-Write capabilities. However, our CI currently runs on ext4. We need absolute certainty that our reflink logic and fiemap ioctls behave correctly on Btrfs without regressions.

Acceptance Criteria:

• Add a step in .github/workflows/ci.yml to create and mount a btrfs loopback device (e.g., using dd and mkfs.btrfs).
• Configure the integration tests to use this mounted Btrfs directory as the sandbox instead of the default /tmp directory.
• Explicitly assert that the engine successfully uses LinkType::Reflink rather than falling back to hardlinks or failing.