Add tamper-evidence: package-DB integrity, self-integrity, dead-man's switch

Answers two hard questions: "what guards the hashes FIM trusts?" and "how do we
make the sensor itself hard to silently disable?" — within the honest limit that
a root attacker who shares your privileges can't be fully stopped on-box, only
made loud.

- pkgdb.py: guards the package DB that `pacman -Qkk` trusts. Anchors a
  fingerprint of /var/lib/pacman/local (refreshed ONLY by the pacman hook) and
  cross-checks pacman.log; a DB change with no logged transaction is flagged
  pkgdb_tamper (CRITICAL, sid 100021) — catches an attacker rewriting a stored
  checksum to mask a modified binary. Verified end-to-end against a simulated
  hash-overwrite.
- selfprotect.py: Sentinel's own binaries/config/units/hook are always in the
  FIM watch set (self-integrity); a heartbeat is written each loop; an external
  `watchdog` command polls a remote dashboard and pushes if the sensor goes
  silent or unreachable — silence becomes the alarm.
- daemon: heartbeat + slow-cadence pkgdb check; DB anchor re-anchored in
  build_fim_baseline so the pacman hook refreshes it after each transaction
- web: /api/status now reports heartbeat_age/stale; dashboard shows it
- cli: pkgdb-check, watchdog (--url/--token/--max-age, bypasses min-severity)
- config: pkgdb_verify/_interval, heartbeat_max_age
- README: threat model (trust-anchor problem) + hardening layers (immutability,
  signed-package + external anchor); reframes "anti-rootkit" as tamper-evidence
- tests: +8 (DB fingerprint, anchor/transaction logic, pacman.log parse,
  heartbeat + watchdog verdicts). 80/80 pass

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

enodia_sentinel/static/dashboard.html

@@ -100,8 +100,14 @@ async function refresh(){
     document.getElementById("host").textContent = st.host || "";
     document.getElementById("ver").textContent = "v"+st.version;
     document.getElementById("ebpf").textContent = "eBPF: "+(st.ebpf||"?");
-    document.getElementById("status").textContent =
-      st.running ? "daemon running" : "daemon NOT running";
+    let hb = "";
+    if (st.heartbeat_age != null)
+      hb = st.heartbeat_stale
+        ? `  ⚠ heartbeat STALE (${Math.round(st.heartbeat_age)}s) — possible tampering`
+        : `  ♥ ${Math.round(st.heartbeat_age)}s ago`;
+    const st_el = document.getElementById("status");
+    st_el.textContent = (st.running ? "daemon running" : "daemon NOT running") + hb;
+    st_el.style.color = (st.running && !st.heartbeat_stale) ? "" : "var(--crit)";
     const c = st.counts||{};
     const cards = document.getElementById("cards"); cards.innerHTML="";
     for(const [lbl,key] of [["CRITICAL","crit"],["HIGH","high"],["MEDIUM","med"]]){