ci: add memtrack walltime benchmarks to CI

[not-matthias]

Adds a memtrack-benchmarks CI job that runs codspeed-memtrack track against a few representative workloads (ls, dd, tar), each with RSS-only and RSS+rmap variants, to catch regressions in memtrack's own overhead.

Runs both walltime (execution time) and memory (memtrack's own peak RSS/allocations while tracking each workload) modes against the same crates/memtrack/codspeed.yml config.

[codspeed]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 17 untouched benchmarks
🆕 6 new benchmarks

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
🆕	WallTime	memtrack track tar	N/A	8.2 s	N/A
🆕	WallTime	memtrack track tar (with physical)	N/A	9.7 s	N/A
🆕	WallTime	memtrack track dd	N/A	1.3 s	N/A
🆕	WallTime	memtrack track dd (with physical)	N/A	1.4 s	N/A
🆕	WallTime	memtrack track ls	N/A	1.1 s	N/A
🆕	WallTime	memtrack track ls (with physical)	N/A	1.2 s	N/A

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_{Comparing cod-3093-memtrack-data-format-improvements (6c953d6) with main (5af022c)}

[greptile-apps]

Confidence Score: 5/5

The pull request appears safe to merge, with no outstanding actionable defects.

Summary

Adds dedicated CI benchmarks that measure the walltime overhead of running memtrack against representative ls, dd, and tar workloads, with and without physical-memory tracking.

• Runs the benchmarks on the dedicated codspeed-macro runner.
• Installs and configures the matching memtrack and CodSpeed runner revisions.
• Batches ring-buffer events to reduce channel-allocation overhead.
• Flushes partial event batches on timeout, explicit drain, and shutdown.
• Updates all internal event consumers to flatten the new batches.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  A[CI workload matrix: ls, dd, tar] --> B[CodSpeed walltime runner]
  B --> C[codspeed-memtrack track]
  C --> D[Tracked workload]
  D --> E[eBPF ring buffer]
  E --> F[Batch up to 1024 events]
  F --> G[Flush on capacity, timeout, drain, or shutdown]
  G --> H[Flatten event batches]
  H --> I[Encode memtrack artifact]

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_{Reviews (5) · Last reviewed commit: "ci: cache runner builds across memtrack ..."}

[GuillaumeLagrange]

olgtm, good idea!

[GuillaumeLagrange]

Memtrack walltime benchmarks took 19m to run, maybe we can shard something before introducing a 20m delay in the CI pipeline? 😇