feat: versioned objects and migrations for syft-client

[bitsofsteve]

Versioned objects for syft-client with migrations, plus peer protocol-schema exchange wired into JobStorage and DatasetStorage.

• VersionInfo (SYFT_version.json): V1 + V2. V2 advertises slim protocol schemas for syft-client/syft-job/syft-dataset; first migrations (v1<->v2).
• ProposedFileChangesMessage (msgv2) and FileChangeEventsMessage: envelopes are the migratable units; items stay unregistered so checkpoint/cache payloads are unchanged.
• Protocol 0 = releases <= 0.1.117 (no identity fields); reads upgrade in memory via shims at the existing choke points. Old clients ignore the new keys (verified against the 0.1.117 models).
• Release discipline: history artifacts for 0.1.117, export script with bump/frozen-once guards, blob fixtures + p2p compat tests.
• PeerManager.live_peer_schemas(protocol) feeds live peer schemas into JobStorage/DatasetStorage: jobs negotiate per peer, datasets per audience. Also fixes peer_schemas or {} dropping the shared reference in both storages.

Out of scope: checkpoints, Peer/SYFT_peers.json, local caches, the semver handshake (unchanged).

One wave per commit; green at every commit.