fix(memtrack): keep mmap/munmap/brk as deprecated artifact variants

[GuillaumeLagrange]

Refs COD-3492

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge; the restored variants match the legacy artifact format and are covered by a focused compatibility test.

Summary

Restores deprecated memory-mapping event variants so artifacts produced by older memtrack versions remain decodable.

• Reintroduces Mmap, Munmap, and Brk with their legacy wire-compatible payloads.
• Adds a MessagePack fixture and regression test proving the stream continues through legacy events to subsequent allocation events.

_{Reviews (1) · Last reviewed commit: "fix(memtrack): keep mmap/munmap/brk as d..."}

[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

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_{Comparing re-add-legacy-events (fd22dda) with main (af3867e)}