Pull Request: eBPF-based CPU Cycle Sampling Module

[dierpg]

Summary
This PR introduces eBPF-based CPU cycle sampling support to efimon via a new SamplingByPIDObserver class and its underlying Sampling_by_PID subproject. It replaces the need for ptrace-based instruction sampling with a lower-overhead eBPF approach that attaches to a target process by PID.

Changes
New: SamplingByPIDObserver
Implements the Observer interface using an eBPF perf-event program to sample CPU cycles from a running process by PID.
Collects instruction pointers and decodes them via the AsmClassifier, producing InstructionReadings.
Supports configurable sampling frequency (Hz) and polling interval; the frequency is automatically capped to the kernel's perf_event_max_sample_rate.
Runs a background thread to periodically drain eBPF ring/perf buffers.

New: Sampling_by_PID subproject
Self-contained eBPF subproject providing the BPF program (prog.bpf.c) and the userspace sampler library (sampler_lib.cpp/.hpp).
Integrated into the efimon build via Meson.
New example: sampling-by-pid-testing
Demonstrates attaching SamplingByPIDObserver to a process, querying samples, and printing instruction mix results.

Build system updates
Added optional eBPF module build paths and linked the new subproject.
Updated include install targets for ebpf-modules/sampling-by-pid and ebpf-modules/moduloio-disk headers.

Housekeeping
Added vmlinux.h to .gitignore.
Cleaned stale files from existing eBPF module directories.
Minor fixes in proc observers, process manager, and tool sources.

Testing
A new example binary sampling-by-pid-testing is provided. Run it against any target PID to verify eBPF attach, sample collection, and instruction classification output.

Notes
Requires kernel support for perf_event_open and eBPF (CONFIG_BPF_SYSCALL, CONFIG_PERF_EVENTS). The observer will fail gracefully if the kernel caps or denies the requested sampling rate.
ObserverScope::SYSTEM is not supported; only ObserverScope::PROCESS is valid for this observer