Fix what the last round of device fixes broke, and two dips it missed

A review of d60ed60 and 8b9b936 found five things. Three were introduced by
those commits.

REFUSING A NAMED DEVICE MADE A MARKER PC CLAIM ITS OPERATION. _sync_partmarker
returned [] both for "not a marker PC" and for "a marker PC that linked
nothing", and the caller reads [] as the first - so a bay whose asset-id.txt
named something unresolvable fell through to the ordinary machine link, and for
a marker PC the machine number IS the operation. It took an active link to a
record that can only have one holder while several markers share it, and the
warning said "not linked". The previous behaviour minted a twin; this traded
that for a contested operation. None now means "not a marker PC" and is the only
answer that lets the machine link run; the response normalises it away so the
API shape is unchanged.

A DORMANT CHALLENGER WAS PROMOTED BY DELETING A FILE. Recording a challenger
dormant leaves a row that the next cycle finds as `reuse` and reactivated with
no incumbent check - so a second PC took a live device by its enrollment file
becoming unreadable. Both reuse branches re-check incumbency now, which is what
_sync_machine_link always did.

AN INCUMBENT UNDER ANOTHER COLLECTOR LABEL WAS INVISIBLE. Incumbency was queried
on our own label, but on a CMM the instrument IS the reported bay, so the
incumbent's link is the machine sync's row. A second PC naming that instrument
found no incumbent and linked actively: two live holders of one instrument, each
invisible to the other. Incumbency now counts any collector-owned label. A row
made BY HAND carries none of them and is still excluded - a person's link is not
the collector's to archive.

THE NETWORK FORM LOCKED OUT THE ROWS IT NEEDED TO FIX. Asset number is disabled
while editing, correctly, but the payload is built in script so the blank was
still sent - and the new server-side guard rejects it. A device with no asset
number could not be saved at all, and the field could not be typed into. It now
unlocks only for a record that loaded without one, with a hint saying why.

"2 in 4.0d" WAS THE LABEL LYING. Replacements are counted across the whole
history window; basisdays is only how long the current cartridge has been in.
Joining them with "in" claimed two changes inside four days - the exact shape
reported as unbelievable, except here the data was right. Now "2, this one 4.0d".

Two toner dips the same review found:

A MULTI-POLL OUTAGE STILL MINTED A PHANTOM SWAP. Only single readings were
dropped, so 90, 0, 0, 90 survived and 0 -> 90 scored as a change. Dips of any
length are handled now. One bad sample stays a candidate whatever the polling
cadence, because a reading is an instant; several consecutive low ones only
count as one outage when they are close together, since days at zero is a real
empty period. That time bound also separates an outage from a swap, ordinary
consumption, and a second swap, which have the same shape in levels alone.

THE DIP FILTER ATE REAL SWAPS OF NEARLY-FULL CARTRIDGES. Recovery was tested
with an absolute difference, so 95 then 5 then 100 read as a recovery because
100 and 95 are close, and the swap evidence was deleted. Toner only falls: a
recovery comes back at or BELOW where it left, a new cartridge comes back
higher.

The review also proved by reverting each feature that the previous tests did not
pin the median burn rate or the near-full rule - both passed with the bug
restored. Verified by the same method that all four toner behaviours now fail
when reverted, and the burst assertion is tight enough to tell 0.2 from 0.88.

plugins/computers/plugin.py

@@ -415,11 +415,13 @@ class ComputersPlugin(BasePlugin):
         partmarkers = self._sync_partmarker(comp, pctype, machinenumber,
                                             warnings, deviceid=deviceid)
 
-        # PC -> machine link from the reported machine number.
-        if partmarkers:
-            machinelinks = []
-        else:
+        # PC -> machine link from the reported machine number. Only a PC that
+        # is NOT a marker PC (None above) takes it: a marker PC reaches its
+        # operation through `partof` on the marker, never by claiming it.
+        if partmarkers is None:
             machinelinks = self._sync_machine_link(comp, machinenumber, warnings)
+        else:
+            machinelinks = []
 
         # Printer relationship sync (only when the payload carried printer data).
         printerlinks = self._sync_printer_links(comp.asset, payload, warnings)
@@ -444,7 +446,10 @@ class ComputersPlugin(BasePlugin):
                 'measuringtoollinkcount': len(measuringtoollinks),
                 'accessprotocols': accessprotocols,
                 'machinelinks': machinelinks,
-                'partmarkers': partmarkers,
+                # `or []`: None is the internal "not a marker PC" signal and
+                # has no business in the response, where it would read as a
+                # different state from "a marker PC that linked nothing".
+                'partmarkers': partmarkers or [],
             },
         }
 
@@ -856,10 +861,21 @@ class ComputersPlugin(BasePlugin):
         if not controls or not deviceasset or not pcasset:
             return True
 
+        # EVERY collector-owned holder, not just ones under this label. On a
+        # CMM the instrument IS the reported bay, so the incumbent's link is the
+        # machine sync's row under MACHINE_LINK_ORIGIN. Filtering on our own
+        # label found no incumbent there, handed the challenger an active link,
+        # and left two live holders of one instrument - each invisible to the
+        # other because they were labelled differently.
+        #
+        # A row made by hand carries neither label and is deliberately NOT
+        # counted: a person's link is not the collector's to archive.
+        ourlabels = (label, MACHINE_LINK_ORIGIN, MEASURINGTOOL_LINK_ORIGIN,
+                     PARTMARKER_LINK_ORIGIN)
         others = AssetRelationship.query.filter(
             AssetRelationship.targetassetid == deviceasset.assetid,
             AssetRelationship.relationshiptypeid == controls.relationshiptypeid,
-            AssetRelationship.label == label,
+            AssetRelationship.label.in_(ourlabels),
             AssetRelationship.isactive.is_(True),
             AssetRelationship.sourceassetid != pcasset.assetid,
         ).all()
@@ -923,7 +939,16 @@ class ComputersPlugin(BasePlugin):
             # Only a device that FILES UNDER an operation goes through here.
             # A measuring tool is a subordinate device too, but it does not
             # share a machine number, so it keeps the simpler path.
-            return []
+            #
+            # NONE, not []. None is the ONLY answer that means "not a marker PC,
+            # run the ordinary machine link". Every other exit below means "this
+            # IS a marker PC" - possibly one that linked nothing - and returning
+            # [] there let the caller fall through to the machine link, which
+            # for a marker PC points at the OPERATION. That is the direct claim
+            # this whole path exists to prevent: several markers serve one
+            # operation, so two marker PCs would contest a link that can only
+            # have one holder, while the warning said "not linked".
+            return None
 
         pcasset = comp.asset
         controls = RelationshipType.query.filter_by(
@@ -998,8 +1023,15 @@ class ComputersPlugin(BasePlugin):
             self._ensure_device_rows(markerasset, spec, pcasset, controls,
                                      label, warnings, active=mayhold)
         elif reuse:
-            reuse.isactive = True
+            # Re-check incumbency HERE too, not only on the named path. A
+            # challenger recorded dormant keeps its row, so the cycle after its
+            # asset-id.txt goes missing found that row as `reuse` and flipped it
+            # active with no incumbent check and no warning - two live holders,
+            # arrived at by deleting a file. _sync_machine_link never had this
+            # hole because it re-checks unconditionally.
             markerasset = db.session.get(Asset, reuse.targetassetid)
+            reuse.isactive = self._device_incumbents_yield(
+                markerasset, pcasset, label, warnings)
         else:
             coretype = AssetType.query.filter_by(
                 assettype=spec['assettype']).first()
@@ -1419,8 +1451,11 @@ class ComputersPlugin(BasePlugin):
                     tooltype.measuringtooltypeid
             targetid = adopted.assetid
         elif reuse:
-            reuse.isactive = True
+            # Same re-check as the marker path: a dormant challenger must not be
+            # promoted just because its enrollment file stopped being readable.
             toolasset = db.session.get(Asset, reuse.targetassetid)
+            reuse.isactive = self._device_incumbents_yield(
+                toolasset, pcasset, MEASURINGTOOL_LINK_ORIGIN, warnings)
             if toolasset and toolasset.measuringtool and tooltype:
                 toolasset.measuringtool.measuringtooltypeid = \
                     tooltype.measuringtooltypeid