fix(repo): send snapshot for shallow-compacted docs

[jacob-petterle]

Summary

• Shallow-compacted docs (truncated oplog) fail to sync to peers with divergent version vectors
• The sync protocol tries delta mode, which silently produces empty data
• Fix: detect shallow docs via shallowSinceVV() and force snapshot mode
• Non-shallow docs are completely unaffected (identical code path)

Root cause: After daemon compacts conversation docs to shallow snapshots, the browser initializes 20 schema ops with its own peer ID. When the daemon tries export({ mode: 'update', from: browserVersion }), the shallow doc can't compute the delta because the browser's peer ops don't exist in the truncated oplog. The protocol marks sync as "synced" with 0 data.

Fix: 5-line change in #buildSyncResponseMessage — detect shallow docs and send full snapshot instead of delta.

const isShallow = docState.doc.shallowSinceVV().toJSON().size > 0
const useSnapshot = isEmpty || isShallow

Risk

Essentially zero.

• Non-shallow docs: isShallow = false → useSnapshot = isEmpty → identical to before
• Shallow docs: sends snapshot instead of broken delta → strictly better
• No data mutation — only changes export mode for read-only sync responses
• shallowSinceVV() is a stable Loro API

Test plan

• Existing handle-sync-request.test.ts tests pass (9/9)
• New test: "should send snapshot (not delta) when document is shallow" — verifies sync-response command is produced for shallow docs with divergent peer versions
• New test: "shallow doc snapshot is importable by fresh browser" — end-to-end proof that a shallow snapshot gives a fresh peer all data

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