Re-architect Sentinel as a zero-dependency Python package

Port the bash prototype to a structured, testable Python codebase while
preserving the same control loop and the seven detection signatures. The bash
implementation stays in src/ as the regression oracle.

Highlights:
- enodia_sentinel/ package (stdlib only — no runtime deps):
  - detectors/ : one pure function per signature, detect(state, cfg)->Alerts
  - system.py  : SystemState — one cached /proc + ss snapshot per sweep,
                 fully injectable so detectors are unit-testable
  - daemon.py  : sweep loop, in-process cooldown dedup, threaded snapshot
                 capture, and a SUID filesystem scan moved OFF the loop
                 thread onto a slow background cadence
  - snapshot.py: forensic text + JSON sidecar with per-signature IR guidance
  - config.py  : dataclass config via TOML + env overrides
  - netutil.py : public-IP / CIDR logic via stdlib ipaddress
- tests/ : 25 stdlib-unittest cases (no root, no /proc, no ss needed)
- TOML config, launcher wrapper, Makefile (pip-free install), hardened
  systemd unit (env-resolved ExecStart), updated PKGBUILD, rewritten README

Performance: per-sweep cost ~200 ms (shared cached state); the multi-second
SUID walk no longer blocks detection. scandir-based walk replaces os.walk.

Verified on Arch: all 7 detectors fire on red-team drills (reverse_shell,
ld_preload, deleted_exe, new_listener, new_suid confirmed live end-to-end),
no false positives on a clean sweep, 25/25 tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

enodia_sentinel/detectors/ld_preload.py

@@ -0,0 +1,46 @@
+"""ld_preload — userland rootkit / library-injection indicators.
+
+Two signatures:
+  * ``/etc/ld.so.preload`` non-empty: injects into *every* dynamically-linked
+    process — the classic system-wide rootkit hook.
+  * a process whose ``LD_PRELOAD`` points into a writable/temp dir — per-process
+    function hooking (hiding files/procs, stealing credentials).
+"""
+from __future__ import annotations
+
+from collections.abc import Iterator
+from pathlib import Path
+
+from ..alert import Alert, Severity
+from ..config import Config
+from ..system import SystemState
+
+_SUSPICIOUS_PREFIXES = ("/tmp/", "/dev/shm/", "/var/tmp/", "/run/user/", "./")
+
+
+def detect(state: SystemState, cfg: Config) -> Iterator[Alert]:
+    preload = Path("/etc/ld.so.preload")
+    try:
+        contents = preload.read_text().strip() if preload.is_file() else ""
+    except OSError:
+        contents = ""
+    if contents:
+        yield Alert(
+            severity=Severity.CRITICAL,
+            signature="ld_preload",
+            key="ldp:global",
+            detail=f"/etc/ld.so.preload is non-empty: [{contents.replace(chr(10), ' ')}]",
+        )
+
+    for proc in state.processes:
+        pre = proc.environ.get("LD_PRELOAD", "")
+        if not pre:
+            continue
+        if pre.startswith(_SUSPICIOUS_PREFIXES):
+            yield Alert(
+                severity=Severity.CRITICAL,
+                signature="ld_preload",
+                key=f"ldp:{proc.pid}",
+                detail=f"pid={proc.pid} comm={proc.comm} LD_PRELOAD=[{pre}]",
+                pids=(proc.pid,),
+            )