feat: add local SpillStore for index scratch

[RitwijParmar]

Summary

This adds the generic local spill mechanism described in #7300 so future memory-budgeted index builders have a session-scoped place for reclaimable scratch files.

Changes include:

• new SpillStore / SpillFile traits and LocalSpillStore implementation in lance-io
• shared DiskQuota accounting wired into local writers for clean cap-exceeded errors
• RAII cleanup for spill files, including quota release on drop
• ScanScheduler::open_reader for already-open readers
• Session wiring with a default local spill store and override hook

This intentionally does not migrate IVF consumers yet; that remains the follow-up slice from #7300.

Validation

• cargo fmt --all
• cargo test -p lance-io spill --lib
• cargo check -p lance-io

I also tried cargo check -p lance, but the local environment is missing protoc, so lance-encoding build script stops before the top-level crate can finish.

Refs #7300

[github-actions]

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