Machine claim vs handover; alerts gated off; ntlars empty-value fix

CHANGELOG.md

@@ -55,10 +55,23 @@ ADR-007 and ADR-002.
   that had been pulled out. The link is a `controls` relationship carrying a
   `collector:machine` origin label, so a link made by hand is never touched.
   Superseding a PC archives the old link rather than deleting it, keeping the
-  history of which PC ran a machine and when, and raises an alert by email and
-  webhook. The retired PC's status is deliberately left alone: the collector
-  cannot tell whether it was shelved, sent for repair or re-imaged for another
-  bay, so a person decides.
+  history of which PC ran a machine and when. The retired PC's status is
+  deliberately left alone: the collector cannot tell whether it was shelved,
+  sent for repair or re-imaged for another bay, so a person decides.
+
+  A second PC reporting a machine another PC already holds is treated as a
+  claim, not a handover. A PC imaged for a machine carries that number before
+  it ever reaches the floor, so the PC actually running the machine keeps the
+  link while it is still reporting and still In Use, and the challenger is
+  recorded as a dormant link. The handover completes on its own once the old PC
+  has been quiet for a day, or immediately when someone moves it off In Use.
+  Without this the two PCs traded the link back and forth on every collector
+  cycle.
+
+  Alerts for both cases go out by email and webhook, gated on a new
+  `computers_machinelink_alerts` setting that ships OFF: a site may legitimately
+  run several PCs on one machine number, and there the alerts fire on correct
+  data. The collector response warns either way.
 - Plugin contract 0.16.0: `get_settings_defaults()` lets a plugin declare the
   settings it owns (key, default, type, category, description, and whether an
   unauthenticated caller may read it). See `docs/PLUGIN-HOOKS.md`.