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/network/frontend/views/NetworkDeviceForm.vue

@@ -24,9 +24,18 @@
                 type="text"
                 class="form-control"
                 :required="isEdit"
-                :disabled="isEdit"
+                :disabled="isEdit && !assetNumberMissing"
                 :placeholder="generatedAssetNumber || 'leave blank to generate from the name'"
               />
+              <!-- Locked while editing, because an asset number is identity and
+                   changing it silently re-keys the record. The exception is a
+                   record that HAS no number: those exist (an update path used to
+                   accept a blank), the server now rejects a blank on save, and
+                   with the field disabled there was no way to supply one - the
+                   row could not be saved at all until it was fixed elsewhere. -->
+              <small class="form-hint" v-if="assetNumberMissing">
+                This device has no asset number. Give it one to save.
+              </small>
               <small class="form-hint" v-if="!isEdit && !form.assetnumber && generatedAssetNumber">
                 Will be created as <code>{{ generatedAssetNumber }}</code>
               </small>
@@ -393,6 +402,10 @@ const generatedAssetNumber = computed(() => {
   if (name.toUpperCase().startsWith(prefix + '-')) return name
   return `${prefix}-${name}`
 })
+// True when the record loaded WITHOUT an asset number, which unlocks the field
+// so it can be repaired. Captured at load: once typed, the field must stay open.
+const assetNumberMissing = ref(false)
+
 const vendors = ref([])
 const models = ref([])
 
@@ -480,6 +493,9 @@ async function loadDevice() {
 
     // Populate form with existing data
     form.value.assetnumber = data.assetnumber || ''
+    // Remember that it arrived empty, so the field stays editable while the
+    // user types the number this record should have had.
+    assetNumberMissing.value = !(data.assetnumber || '').trim()
     form.value.name = data.name || ''
     form.value.serialnumber = data.serialnumber || ''
     form.value.gaugelabreference = data.gaugelabreference || ''