Add local read-at support for compio retrieval

[danking]

Authored by Codex.

Summary

Add the Vortex-side APIs needed for Spiral's compio/cyper object-storage client path.

• Add a local, non-Send read-at interface alongside the existing Send interface.
• Add OpenOptions::open_read_local so footer discovery can run on a caller-selected local I/O worker.
• Keep each local file's segment retrieval driver pinned to the same LocalIoWorker; the driver's cyper/compio state machine never moves between threads.
• Preserve Vortex's existing segment request queue and completion callbacks as the retrieval boundary.
• Keep scan tasks themselves Send. Query workers await scan tasks and perform pruning, decode, and projection; only segment retrieval executes on the compio worker.
• Remove the earlier scan-level local task API after review showed it placed CPU/query processing on I/O event-loop threads.

This is the Vortex dependency for Spiral draft PR https://github.com/spiraldb/spiraldb/pull/6406.

Design

The structure follows the retrieval/query-worker split in sections 3 and 4 of Exploiting Cloud Object Storage: a small number of retrieval event loops own many state-machine requests, while query workers process completed morsels. File affinity is intentional because compio/cyper futures are non-Send; hostname affinity is not used.

Validation

• cargo +nightly-2026-06-29 check -p vortex-io -p vortex-layout -p vortex-file
• Spiral's strict integration test opens a generated 4,096-row Vortex file through open_read_local, materializes every row, verifies only local provider methods were called, and verifies scan map/decode work did not execute on vortex-io-* threads.

This remains draft while the dependent Spiral branch is measured on Linux io_uring.