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/new_suid.py

@@ -0,0 +1,35 @@
+"""new_suid — a SUID/SGID binary that wasn't in the baseline.
+
+A new setuid binary is a privilege-escalation persistence trick; one in a
+world-writable dir (``/tmp``, ``/home`` …) is almost never legitimate, so it is
+escalated to CRITICAL. The filesystem walk is expensive, so the daemon only
+populates ``state.suid_binaries`` on its slow scan cadence — when it's absent
+this detector is a no-op.
+"""
+from __future__ import annotations
+
+from collections.abc import Iterator
+
+from ..alert import Alert, Severity
+from ..config import Config
+from ..system import SystemState
+
+
+def detect(state: SystemState, cfg: Config) -> Iterator[Alert]:
+    if state.suid_binaries is None or state.suid_baseline is None:
+        return
+    for path in state.suid_binaries:
+        if path in state.suid_baseline:
+            continue
+        hot = any(
+            path.startswith(d.rstrip("/") + "/") for d in cfg.suid_hot_dirs
+        )
+        yield Alert(
+            severity=Severity.CRITICAL if hot else Severity.HIGH,
+            signature="new_suid",
+            key=f"suid:{path}",
+            detail=(
+                ("SUID/SGID binary in writable dir: " if hot
+                 else "new SUID/SGID binary: ") + path
+            ),
+        )