Retire the dead v1 MAP_SHARED/Msync snapshot surface (ADR 0045 D1)

[nikhilunni]

The engrams live-migration fork sits on upstream v1.16.0 + three commits:

• v1 (5ca2fba4) — MAP_SHARED memory + Msync/MsyncAndState off-pause flush
• v2 (4a9636d0) — MAP_PRIVATE base-file + MISSING|MINOR substrate
• v3 (d78c1dc1) — vmstate_only create param

v1's MAP_SHARED/msync design was superseded by the v2 UFFD substrate + the v3 post-copy path and is dead: nothing emits it (the engrams host's SnapshotType is {Full, Diff}; there's no shared load param on the host side). engrams ADR 0045 D1 marked it for retirement.

What this removes

A revert of 5ca2fba4 reconciled with v2/v3:

• SnapshotType::{Msync, MsyncAndState}
• the shared load param (LoadSnapshotParams/LoadSnapshotConfig + swagger)
• vstate/memory.rs msync() + the shared MAP_PRIVATE→MAP_SHARED flag on snapshot_file (back to the upstream 3-arg, MAP_PRIVATE signature)
• vstate/vm.rs msync dispatch in snapshot_memory_to_file
• persist.rs Msync state-file skip + the now-vacuous vmstate_only-vs-Msync guard (vmstate_only combines only with Full/Diff, the only remaining types)
• the api_server + rpc_interface surface; the integration test for the dead guard

9 files, 24 insertions / 163 deletions.

Preserved / invariants

• v2 (substrate) and v3 (vmstate_only) are intact. The v2 base-file path that called snapshot_file(.., shared=false) now uses the reverted 3-arg signature (MAP_PRIVATE — its existing behavior).
• No change to src/vmm/src/snapshot/ and no SNAPSHOT_VERSION bump — byte-compatible with stock, the invariant the v1 commit itself established.

Validation

cargo check -p vmm (lib and tests) for aarch64-unknown-linux-musl is clean. The firecracker binary crate isn't musl-checkable locally (the upstream seccompiler crate has no libc::memfd_create on musl), so the full build + the stock↔fork snapshot-compat test are left to CI / the engrams build-firecracker step. The api_server/swagger changes are mechanical field-drops matching the validated vmm struct change.

Once green, the engrams repo bumps its third_party/firecracker submodule to this commit (separate PR).

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