feat: drop the psutil dependency and fix documentation divergences

[JeanExtreme002]

Summary

Makes the core library truly dependency-free by replacing psutil with native per-platform process discovery, and corrects a set of documentation/docstring claims that did not match the actual runtime behavior.

Drop psutil

psutil was the only hard runtime dependency, used solely to resolve a process name to a PID and to check whether a PID exists. Both are now done natively:

• Windows — CreateToolhelp32Snapshot(TH32CS_SNAPPROCESS) + Process32First/Next; pid check via OpenProcess(PROCESS_QUERY_LIMITED_INFORMATION).
• Linux — enumerate /proc/<pid>, read the name from /proc/<pid>/comm; pid check via /proc/<pid>.
• macOS — libproc proc_listpids + proc_name (basename of proc_pidpath as fallback); pid check via os.kill(pid, 0).

process.util now dispatches to the per-OS backend. psutil moves from a required dependency to the [app] extra — the desktop GUI's process picker still uses its richer username/memory info, which is beyond what the core needs.

Process-name matching semantics (case_sensitive, exact_match) are preserved. On Linux the name comes from comm, which the kernel truncates to 15 characters — the same limitation psutil had here.

Documentation/docstring fixes

• DEFAULT_MAX_REGION_CHUNK is 256 MiB (was documented as 16 MiB).
• iter_region_chunks chunks are contiguous, not overlapping (pattern-boundary handling happens in the scanner).
• Partial reads raise OSError — they are not logged as WARNING.
• ThreadInfo.start_address is always None on every backend (documented as reserved).
• RemotePointer bufflength is required only to read str/bytes; writing accepts None.
• Corrected the compile_pattern example output.
• OpenProcess signature now shows the real platform-specific defaults.
• macOS ModuleInfo name/path are both empty when the image path can't be resolved (the fallback only applies on Windows).
• read_string reads exactly byte_count bytes (raises on a short read), not "up to".
• ptr_size defaults to None (auto-detected from target bitness) across the pointer APIs.
• Migrated search_by_value / search_by_addresses examples to keyword arguments.

Testing

• New tests/test_process_enumeration.py exercises the native enumeration and pid_exists on the host OS.
• Existing lookup tests rewired off psutil.
• Full non-GUI suite passes (312 passed, 13 skipped). The Windows/Linux native paths are covered by CI on those platforms, as with the existing thread/module enumeration tests.