[beinan]

Summary

Add an optional Linux Rust sampler for an existing experiment operation. It validates the recorded process identity and cgroup scope, reports process/container resource counters and optional allocated WAL blocks, and completes when that incarnation exits. Permission denial alone never counts as process completion.

The sampler is read-only. Documentation distinguishes sampled occupancy from hard bounds and explains observation intervals; CI exercises the optional feature in a private cgroup namespace with privileges dropped before the integration test.

Refs #378.

Testing

• Five sampler unit tests and one real-process lifecycle test passed, including real procfs permission transitions for exiting and still-live targets.
• Four existing runner process regressions passed. Both ignored fixtures are invoked by their tests.
• Optional-feature release Clippy, formatting, spelling and whitespace checks passed.
• Workspace formatting, Clippy and all-target Rust tests passed (594 tests). After building the current server, Python tests passed with CI mode enabled (229 passed, 2 skipped, 1 xfailed); Ruff and Pyright passed.
• The process integration test passed in the local container namespace. Hosted-runner namespace setup is exercised by CI.