compute: import published indexes as a shared arrangement

[antiguru]

Fifth of eight PRs splitting #37770. Stacks on #38389. Tracked by CPU-215.

ArrangementFlavor gains a SharedTrace variant carrying arrangements backed by TraceFrontier<SharedTraceHandle>, and import_index_shared constructs it on the interactive runtime instead of reading the local TraceManager, which holds nothing there. The shared handle shares the RowRow/Err batch and cursor types of a maintenance TraceAgent, so the variant flows through the same generic bodies as Trace and a downstream Get receives a real arrangement with its key and permutation intact, rather than a collection the plan re-derives. Joins, delta joins, thresholds, and hydration logging gain the corresponding arm.

The import is a static snapshot at as_of, bounded one step past it rather than by self.until: interactive work is single-time, so the capability must drop once the shared trace seals past as_of for the one-shot result to complete, and an unbounded until never gets there.

The read hold is the returned Arranged's own trace rather than a separate token, so consumers can downgrade it as their frontiers advance. A token nobody downgrades would be a floor under every hold that is downgraded, since the publisher forwards the meet.

The variant and its constructor land together because a never-constructed variant of an enum inside the private render module trips dead_code, and CI lints with -D warnings. Reachable only when a runtime holds the Interactive role, which nothing constructs yet.