feat(server): Add configurable memory balloon for MicroVMs

[dpletter]

Description

Adds an optional balloon block to a pool's firecracker config that attaches Firecracker's memory balloon device to each MicroVM before boot.

• New FirecrackerBalloonConfig with amount_mib, deflate_on_oom, stats_polling_interval_s and free_page_reporting. It is validated (no negative values). Without the block, MicroVMs are created exactly as before.
• The device is attached with a direct PUT /balloon on the Firecracker API socket, added to the FcInit handler list. The SDK's balloon model has no free_page_reporting field, so it cannot express the option that makes the balloon useful here.
• With free_page_reporting: true, memory the guest frees goes back to the host continually. Without it, a MicroVM keeps its peak memory until it exits.
• The configuration reference documents each option and the guest kernel requirements (CONFIG_VIRTIO_BALLOON, and CONFIG_PAGE_REPORTING for free page reporting).

Verified on Firecracker 1.16.1 with a 6.1 guest kernel. After the guest filled and freed 1.2 GB, host RSS of the Firecracker process fell from 1277 MiB to 83 MiB within 10 seconds; with no balloon it stayed at 1277 MiB. It has run on a production CI host with two pools since 2026-09-26.

Related Issue(s)

Closes #347

Checklist

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• I have added necessary documentation (if appropriate).
• I have added tests to cover my changes.