Schema column loading is not cancelled when a navigation is superseded #2058

Clicking through the sidebar quickly wastes 75-90ms per navigation on schema-column loading that is thrown away, and the tables in between never load.

Evidence

From a live trace on a Postgres connection over an SSH tunnel, clicking ten tables in a row at roughly 85ms intervals:

#4 beta_signups  +24.7ms  schemaColumnsBegin
#4 beta_signups  +84.0ms  ANOMALY supersededByNewNavigation replacedBy=#5   <- next click
#4 beta_signups  +100.6ms schemaColumnsEnd                                   <- still ran the full 76ms
#4 beta_signups  +100.6ms END outcome=prepareAbandoned

Identical for #5, #6, #7, #8, #9, #10, #11, #12 and #13. Ten consecutive navigations each burned 75-90ms of schema loading and then discarded it. Only #14 survived to display anything.

Why it happens

loadSchemaColumns runs inside prepareTableTabFirstLoad (MainContentCoordinator+TableFirstLoad.swift), which sits before executeStarted. Two consequences:

• The execution claim added in fix(coordinator): give each tab ownership of its execution so a retarget cannot be overwritten #2055 does not cover it. A claim is minted at executeQueryInternal, so work before that point has no owner to invalidate.
• cancelTableLoad cancels the wrapper Task in tableLoadTasks, but the await loadSchemaColumns(...) inside it is not cancellation-aware, so it runs to completion regardless.

The post-await guard then correctly discards the result, which is why the outcome is prepareAbandoned rather than a wrong-table bug. Nothing is incorrect here; it is purely wasted work.

Pre-existing

Not introduced by #2055. The trace added there is simply the first thing that made it measurable.

Suggested direction

Either extend the claim to cover preparation (mint at openTableTabQuery rather than executeQueryInternal, and check isCurrent before and after the await), or make loadSchemaColumns honour Task.isCancelled at its suspension points. The former also gives the preparation phase a timeline entry that the trace can attribute.