fix(#242): ship rootfs as content-addressed sidecar so hub packs are portable

[WaylandYang]

Closes #242.

The bug

Hub snapshot packs couldn't restore on any host but the one that baked them. Firecracker freezes the rootfs's absolute path into the vmstate and reopens it verbatim at restore, but from-image bakes keep the rootfs at /var/cache/forkd/<image>.ext4 — outside the snapshot dir — so it was never packed, and snapshot.json didn't even record which rootfs. First fork on another host died with FC's cryptic Block: Error manipulating the backing file: No such file or directory. This almost certainly explains the near-zero hub asset usage (everyone who tried forkd pull + fork on their own host bounced).

The fix — sidecar + dedup

The rootfs ships as a content-addressed <sha256>.rootfs.zst sidecar beside the pack, not inside it — so the memory-delta pack stays ~16 MiB (mostly-zero RAM, 33× compression) while the rootfs (real binaries, ~12–50× compression) travels separately and dedupes: packs sharing a base rootfs reference one sha-named asset, and the puller caches by sha.

• forkd-vmm: Snapshot records rootfs (the path FC froze); written by snapshot/from-image, inherited through compact. #[serde(default)] — old snapshot.json stays valid.
• pack: emits the sidecar at zstd -19 (worth it — compressed once, downloaded by all), records RootfsRef { target_path, sha256, size } in the manifest. Additive field, no pack-version bump.
• pull / unpack: places the rootfs at the recorded absolute path, skips when already present with a matching sha. Local unpack reads the sibling file; pull fetches the pack-URL sibling. Atomic (temp + rename) + sha-verified.
• Graceful degradation: an unsatisfiable sidecar warns at pull time with an actionable message instead of the cryptic block error at first fork.
• push (single presigned PUT can't place a sibling) warns that the sidecar must be uploaded alongside; the hub-publish flow is forkd pack (emits both files) + a release upload of both.

Verified end-to-end

Isolated privileged container (host networking untouched), full portability cycle:

      "vmstate": "/root/.local/share/forkd/snapshots/quickstart/vmstate",
      "memory": "/root/.local/share/forkd/snapshots/quickstart/memory.bin",
      "volumes": [],
      "rootfs": "/var/cache/forkd/python-3-12-slim.ext4"
    }
    ===== 3. pack → should emit .tar.zst + <sha>.rootfs.zst sidecar =====
    ==> packing snapshot 'quickstart' → /tmp/pk/qs.tar.zst
    ==> compressing rootfs sidecar 0abd0245d267a844.rootfs.zst (zstd -19, 1.50 GiB → once per base)...
        1.50 GiB → 29.1 MiB (52.8× ) in 29.5s
    ✓ wrote 15.5 MiB (512.0 MiB uncompressed; 33.0× compression) in 31.3s
      next: scp/upload, then `forkd unpack /tmp/pk/qs.tar.zst` on the target host
    --- /tmp/pk contents: ---
    total 45688
    drwxr-xr-x 2 root root     4096 Jun 14 10:37 .
    drwxrwxrwt 1 root root     4096 Jun 14 10:36 ..
    -rw-r--r-- 1 root root 30491184 Jun 14 10:37 0abd0245d267a844.rootfs.zst
    -rw-r--r-- 1 root root 16281099 Jun 14 10:37 qs.tar.zst
    
    ===== 4. simulate OTHER HOST: wipe rootfs + snapshot =====
    rootfs gone: ls: cannot access '/var/cache/forkd/*.ext4': No such file or directory
    
    ===== 5. unpack → should place rootfs from sidecar =====
    ==> unpacking /tmp/pk/qs.tar.zst ...
    ✓ unpacked tag 'quickstart' at /root/.local/share/forkd/snapshots/quickstart
      next: forkd fork --tag quickstart -n <N>
    ==> placing rootfs → /var/cache/forkd/python-3-12-slim.ext4 (from /tmp/pk/0abd0245d267a844.rootfs.zst)
    ✓ rootfs ready at /var/cache/forkd/python-3-12-slim.ext4
    --- rootfs restored? ---
    -rw-r--r-- 1 root root 1610612736 Jun 14 10:37 /var/cache/forkd/python-3-12-slim.ext4
    
    ===== 6. fork from the unpacked snapshot (proves portability) =====
    ==> forking 2 children from snapshot 'quickstart' (per-child netns)...
    ✓ all sockets up in 50 ms
    ✓ 2 restores fired in parallel in 6 ms
    ✓ total wall-clock: 56 ms
    ==> letting children settle for 2s...
    ✓ 2 / 2 children alive
    ==> shutting down...
        cleaned work_dir /tmp/forkd-fork-quickstart
    EXIT=0

Bake → pack (1.5 GiB rootfs → 29 MiB sidecar, 52.8×; main pack stays 15.5 MiB) → wipe rootfs+snapshot (simulate another host) → unpack (rootfs auto-placed) → fork 2/2 alive in 56 ms.

Scope / follow-ups

• Bases (from-image / snapshot — the packable things) carry the rootfs. Daemon-side branches set rootfs=None for now (not a hub path) — branch inheritance is a clean follow-up.
• Existing hub assets (python-numpy etc.) still need a re-bake + re-pack + re-upload of both files to become portable — an ops task, tracked separately. This PR fixes every pack produced from here on.

Test plan

• forkd-cli 23 + forkd-vmm 39 unit tests pass (incl. new path-rewrite + round-trip)
• cargo clippy --workspace --all-targets -D warnings + cargo fmt --check clean
• Full portability E2E (above)
• CI green

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