Stream a table's change events

Server-sent event stream of a table's changes: row cell deltas, run progress, and column / display-preference / row-set invalidation signals. Replaces the 2-second status poll, so an idle table costs nothing.

Deltas carry the row's version, the same watermark the rows list carries, so applying one twice or out of order is a no-op. There is deliberately NO replay log: clients refetch once on every connect, and a mid-stream gap is reported as a resync frame meaning the same thing.

The server closes the stream after a few minutes with a reconnect frame; reconnect with a fresh token. Comment heartbeats keep intermediaries from dropping an idle connection.

Requires Clerk authentication; API keys are rejected. There is deliberately no 404: an unknown or foreign table streams heartbeats only, because the channel is org-scoped and leaks nothing.