storage: avoid B-tree cache rescans

[Jordan Olshevski (jveski)]

Carefully mutate cached branches of the btree instead of re-reading everything from disk. Reduces small random reads significantly.

After this change, the btree store operates within ~3% of line rate for typical nvme devices.

[Copilot]

Pull request overview

This PR improves unbounded-storage B+tree commit performance by updating the in-memory internal-node cache incrementally after path-copy commits, rather than rescanning and re-reading internal nodes from the block device. It also shifts some mutator bookkeeping lookups to use the committed in-memory mirror to avoid extra device reads and reduce fault-injection sensitivity.

Changes:

• Update internal-node cache by reusing cached internal nodes and inserting newly encoded internal nodes, avoiding full cache rebuild rescans.
• Switch mutator prior/current entry probes to use lookup_committed_mirror (no device reads) and add targeted assertions that no reads occur in those paths.
• Add/adjust tests and invariants documentation to reflect the new corruption/eviction accounting and I/O behavior.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
cmd/unbounded-storage/tests/storage/tests.rs	Updates invariant rationale/message to reflect that prior-LBA probes now use the committed mirror rather than disk reads.
cmd/unbounded-storage/src/storage/engine.rs	Uses lookup_committed_mirror for mutator bookkeeping and adds a test asserting an eviction probe causes zero device reads.
cmd/unbounded-storage/src/storage/btree/tests.rs	Adds a test asserting commits update the internal cache without rescanning, using MockDevice read-count deltas.
cmd/unbounded-storage/src/storage/btree/mod.rs	Introduces incremental internal-cache update usage in apply_batch and refines lookup_committed_mirror/cache-bytes documentation.
cmd/unbounded-storage/src/storage/btree/cow.rs	Stores internal cache nodes as Arc and adds update_internal_cache fed by internal nodes produced during path-copy encoding.

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