feat(memtrack): add zstd compression support

[not-matthias]

Changes in this PR:

• Writing files to disk directly after reading from eBPF (reduced the overall overhead from 20s to 3s on a large example)
• Zstd compression (which decreases the file size by ~10x)

[Copilot]

Pull request overview

This PR adds zstd compression support to memtrack artifacts to reduce storage requirements. The implementation introduces streaming compression/decompression and refactors the event processing architecture to write events directly to disk during collection rather than accumulating them in memory.

Key changes:

• Introduces MemtrackWriter with zstd compression for streaming event serialization
• Refactors track_command to use a two-stage threading model (drain thread → writer thread) for immediate disk writing
• Centralizes filename generation logic in the ArtifactExt trait

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
crates/runner-shared/src/artifacts/mod.rs	Adds file_name() method to centralize filename generation for artifacts with optional PID
crates/runner-shared/src/artifacts/memtrack.rs	Introduces MemtrackWriter struct with zstd compression and updates decoder to decompress streams
crates/memtrack/src/main.rs	Refactors event processing to use two-stage threading (drain + writer) for streaming events to compressed disk files
crates/runner-shared/Cargo.toml	Adds zstd dependency version 0.13
Cargo.lock	Updates lock file with zstd and its transitive dependencies

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[codspeed-hq]

CodSpeed Performance Report

Congrats! CodSpeed is installed 🎉

🆕 4 new benchmarks were detected.

You will start to see performance impacts in the reports once the benchmarks are run from your default branch.

Detected benchmarks

• write_events[1000000] (Simulation): 2.1 s
• write_events[100000] (Simulation): 202.5 ms
• write_events[500000] (Simulation): 1 s
• write_events[10000] (Simulation): 19.5 ms

[Copilot]

Pull request overview

Copilot reviewed 6 out of 7 changed files in this pull request and generated 4 comments.

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[Copilot]

The file extension .msgpack is misleading now that the files are compressed with zstd. Consider changing the extension to .msgpack.zst or .msgpack.zstd to accurately reflect that the files are compressed, which will help users understand the file format and ensure proper handling.

Suggested change

	format!("{pid}.{}.msgpack", Self::name())
	} else {
	format!("{}.msgpack", Self::name())
	format!("{pid}.{}.msgpack.zst", Self::name())
	} else {
	format!("{}.msgpack.zst", Self::name())

[not-matthias]

I agree that it's misleading, but I think it's not worth it changing the file extension. The data format is hidden in the MemtrackWriter and the MemtrackEventStream, so the consumer doesn't care about it.