feat(orch): re-enable memory.peak per-FD reset (cgroups v2)

[arkamar]

Now that we run on kernel 6.12+ (ubuntu24), re-enable per-FD peak reset for memory.peak. Each GetStats() call reads the interval peak and writes "0" to reset it, giving per-sample granularity instead of lifetime-only tracking.

Restores: 8cf9d74 ("fix: disable memory.peak per-FD reset (requires kernel 6.12+) (#1959)")

[cursor]

PR Summary

Medium Risk
Changes low-level cgroup stats collection to write-reset memory.peak on each sample, which can affect resource accounting accuracy and may behave differently across kernel/config variations.

Overview
This PR switches cgroups v2 memory.peak tracking from a lifetime peak to an interval peak by keeping a writable memory.peak FD open, reading it during GetStats(), and writing to it to reset the counter for the next sample. It also tightens parsing/error handling, adjusts logging to surface when reset isn’t available, updates the host stats comment to reflect interval semantics, and revises the cgroup integration test to validate per-sample peak behavior rather than monotonic lifetime growth.

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