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/alert.py

@@ -0,0 +1,39 @@
+"""Alert and severity types shared by all detectors."""
+from __future__ import annotations
+
+import enum
+from dataclasses import dataclass, field
+
+
+class Severity(enum.IntEnum):
+    MEDIUM = 1
+    HIGH = 2
+    CRITICAL = 3
+
+    def __str__(self) -> str:  # pragma: no cover - trivial
+        return self.name
+
+
+@dataclass(frozen=True)
+class Alert:
+    """A single detection.
+
+    `key` is a stable dedup identity (e.g. ``rsh:1234``) used by the daemon's
+    cooldown so the same condition doesn't re-fire every sweep. `pids` lists
+    processes the snapshot should deep-dive.
+    """
+
+    severity: Severity
+    signature: str
+    key: str
+    detail: str
+    pids: tuple[int, ...] = field(default_factory=tuple)
+
+    def to_dict(self) -> dict:
+        return {
+            "severity": str(self.severity),
+            "signature": self.signature,
+            "key": self.key,
+            "detail": self.detail,
+            "pids": list(self.pids),
+        }