Multi-partition inverted-index reads (FTS prewarm, corpus stats, df)
fan out over partitions through one shared ScanScheduler. Every
partition opened its files at base_priority 0, so all concurrent reads
tied at the same priority.

The scheduler's backpressure deadlock-break admits the lowest-priority
in-flight request without a byte-budget check, which requires a total
order over in-flight priorities to guarantee one request can always
advance and drain the byte budget. With every partition at priority 0
there is no unique lowest request, so when the shared 64-IOP scheduler
saturates, freed IOP slots go to arbitrary tied tasks and no consumer's
batch is guaranteed to complete and refund resources -> the I/O loop
can wedge with work pending but none admissible.

Stamp each partition's IndexStore with its enumerate index as the base
priority at load time, mirroring how a filtered read scan prioritizes
each fragment. The stores share the same scheduler (Arc), so global
backpressure is unchanged; only the priority differs, restoring the
total order across partitions that the deadlock-break relies on. The
single, non-concurrent metadata read stays at priority 0 (no tie).

Adds IndexStore::with_base_priority (default no-op for backends without
a priority concept) and a test asserting partitions load with distinct,
dense priorities.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

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