Rename liveCacheBuster to cursor/serverCursor in ShapeStreamState

## Summary

`liveCacheBuster` in `SharedStateFields` (packages/typescript-client/src/shape-stream-state.ts) is misleadingly named. It's not a cache buster — it's the server-assigned cursor from the `electric-cursor` response header, used as the `live_cache_buster` query parameter.

The name creates confusion when reading the state machine code because:
- "Cache buster" implies a random/rotating value used to bypass caches
- It's actually a server-assigned cursor that identifies a position in the change stream
- There's a separate `staleCacheBuster` field that IS an actual cache buster (randomly generated)

## Suggestion

Rename `liveCacheBuster` to `cursor` or `serverCursor` throughout the state machine. The query parameter name (`live_cache_buster`) is a server protocol concern and can stay as-is in the URL construction.

## Affected code

- `SharedStateFields.liveCacheBuster` → `cursor` / `serverCursor`
- All references in `shape-stream-state.ts`, `client.ts`, tests, and SPEC.md
- The constant `LIVE_CACHE_BUSTER_QUERY_PARAM` can keep its name (it maps to the protocol)

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