fix(memtrack): handle dropped events better#321
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The eBPF ring buffer was 256 KiB (~4.7k events). High-allocation benchmarks could fill it faster than userspace drained, silently losing events. Bump it to 16 MiB (~300k events) and add a single-slot BPF array that bpf_ringbuf_reserve() failures atomically increment, so userspace can detect when the trace is incomplete. Refs COD-2577.
Replace the 10ms-timeout poll loop with a blocking epoll wait that combines the ring buffer's epoll fd and a shutdown eventfd. The kernel now wakes the drain thread the moment events arrive, which removes the polling-induced backlog under bursty workloads. Refs COD-2577.
The streamed event file has no header, so per-run summary data has
nowhere to live. Add a small JSON sidecar (`{pid}.MemtrackMeta.json`)
carrying the dropped-event count so the backend can invalidate
benchmarks whose trace is incomplete.
Refs COD-2577.
After the tracked command exits, read the eBPF dropped-event counter, log a warning if non-zero, and persist the value as a MemtrackMeta sidecar next to the events file. Lets the backend mark a benchmark as incomplete instead of silently accepting a truncated memory trace. Refs COD-2577.
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