perf(bench): make hot_path compile by default, representative and profile-able (LAB-7039)

[27Bslash6]

Makes the hot_path Criterion bench usable for instruction-count profiling and closer to real workloads.

• required-features = ["encryption"] on the bench target: a plain cargo bench now skips hot_path instead of failing to compile. make bench is unchanged.
• [profile.bench] with strip = false and line tables. The release profile's strip = "symbols" also discarded the line tables, so profilers saw only addresses.
• All fixtures, including the 64 MiB incompressible setup, build lazily inside the routine closure. Criterion calls only closures whose id matches the filter, so a filtered run no longer pays for the 64 MiB setup.
• New byte_storage/roundtrip_msgpack/<size> (msgpack-encoded records) and byte_storage/roundtrip_incompressible/<size> ids sit beside the original byte_storage/roundtrip/<size> ids, which keep their history. Each corpus prints its LZ4 compressed/raw ratio once (at 64 KB: ramp 0.008, msgpack 0.377, incompressible 1.004).
• examples/bench_throughput.rs prints the OS/arch instead of a hard-coded machine name.

README's Performance section now says how to run the suite and what the corpora are.

Checks: cargo bench --no-run (no features) exits 0; all 28 ids pass cargo bench --features encryption --bench hot_path -- --test; cargo test --features encryption, cargo clippy --features encryption --all-targets -- -D warnings and cargo fmt --check pass.

Closes LAB-7039

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📥 Commits

Reviewing files that changed from the base of the PR and between 47af1fb and 2d17ca9.

📒 Files selected for processing (4)

• Cargo.toml
• README.md
• benches/hot_path.rs
• examples/bench_throughput.rs

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Summary by CodeRabbit

• Performance

  • Benchmark coverage now includes realistic MessagePack and incompressible data, alongside the existing synthetic data, giving a broader view of storage round-trip performance.
  • Benchmark results include compression ratios for the additional data types and identify the host operating system and architecture.
  • Profiling tools can provide more detailed attribution when analysing benchmark performance.

• Documentation

  • The performance guide now describes the benchmark data and the feature required to run the suite.

Walkthrough

The benchmark profile retains symbols and line tables, and the hot-path benchmark requires the encryption feature. Round-trip benchmarks now cover synthetic ramp, realistic MessagePack, and incompressible payloads. Large envelope fixtures and encryption plaintext are created lazily.

Changes

Benchmark suite

Layer / File(s)	Summary
Benchmark setup and reporting Cargo.toml, README.md, examples/bench_throughput.rs	The bench profile retains symbols and line tables, and the hot-path benchmark requires the encryption feature. The README describes the benchmark corpora and profile. The throughput header reports the host operating system and architecture.
Round-trip corpus benchmarks benches/hot_path.rs	The benchmark creates MessagePack and incompressible payloads, reports their compression ratios, and runs round-trip benchmarks for these payloads and the synthetic ramp.
Lazy benchmark fixtures benches/hot_path.rs	The 64 MiB envelope fixture and encryption plaintext are initialised inside benchmark closures. Envelope encode, decode, store, and retrieve benchmarks use the shared fixture.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Feature

Merge Risk: ⚪ Minimal · up to 2d17c

The changes broaden benchmark workloads and improve profiling and filtered execution without an established functional regression. No merge-blocking issue remains, subject to normal checks.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 36.36% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 11 functions across 2 files. (2 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarises the main benchmark changes: default compilation behaviour, representative workloads, and profiler support.
Description check	✅ Passed	The description directly explains the benchmark, profiling, lazy fixture setup, added corpora, documentation updates, and validation results.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

Docstring coverage is 36.36% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 11 functions across 2 files. (2 skipped: 2 unsupported.)

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