Add crucible-downstairs validate subcommand

[mkeeter]

This is a spin-off from #1791; it adds validation to crucible-downstairs but does not validate extent after receiving them during repair operations.

[Copilot]

Pull Request Overview

This PR adds a new verify subcommand to crucible-downstairs that validates the integrity of region data by checking that block hashes match their stored context slots.

Key Changes:

• Adds verify subcommand to the crucible-downstairs CLI
• Implements validation logic for raw extents that compares block data hashes against stored context slots
• Validates in parallel using rayon for improved performance

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
downstairs/src/main.rs	Adds Verify subcommand enum variant and routes it to verify_region function
downstairs/src/lib.rs	Exports new verify_region function from dump module
downstairs/src/extent.rs	Adds validate trait method and public wrapper for extent validation
downstairs/src/extent_inner_raw.rs	Implements validation logic for raw extents by reading and hashing blocks in bulk
downstairs/src/extent_inner_sqlite.rs	Stubs out unimplemented validation for Sqlite extents
downstairs/src/dump.rs	Implements verify_region function with parallel validation using rayon

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