Example optimization hunt

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What does this PR do?

Implements a zero-copy optimization for Cache::read_at in the block cache that significantly improves performance for reads that fit within a single block. When a read is contained within one block, the function now returns a slice of the existing Bytes instead of allocating a new buffer and copying data.

Motivation

This optimization dramatically improves performance for common read patterns in the logrotate_fs file generator:

• 86.9% reduction in time for 1KiB reads (36.7ns → 4.8ns)
• 99.5% reduction in time for 64KiB reads (1.02μs → 4.9ns)
• No change for 1MiB reads that span multiple blocks (expected)

Related issues

N/A

Additional Notes

The PR also documents two optimization attempts in the optimization database:

1. The successful zero-copy optimization for Cache::read_at
2. A failed buffer reuse optimization for trace_agent::v04::V04::to_bytes that reduced memory allocation by 65.6% but showed no timing improvement because serialization cost dominates

The zero-copy approach works well because:

1. Block data is already stored as immutable Bytes (reference-counted)
2. Most reads are smaller than block size (256KiB blocks, typical reads 1-64KiB)
3. The fast path check is a simple comparison
4. The slow path (multi-block reads) maintains the original behavior

The full Claude run can be found here: https://drive.google.com/file/d/1mbgFAFuC5s2HwM3IhoNCgqZcmt7Ho7a2/view?usp=sharing

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Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Example optimization hunt #1762 👈 (View in Graphite)
• Update hunt to use targetted benchmarks #1761
• Optimize benchmarks #1760
• Split benchmarks into individual targets #1759
• main

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Closing this as I don't want to merge these optimizations/optimization hunt results in yet (I've got more changes coming afterwards to make the reports consistent).

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In case this gets missed in the PR description, the full claude run can be seen here: https://drive.google.com/file/d/1mbgFAFuC5s2HwM3IhoNCgqZcmt7Ho7a2/view?usp=sharing