docs: ADR-018 supersedes ADR-005, metrology instruments are machines

ADR-005 split machines from measuring tools on the grounds that they are two
asset classes. It named the metrology population as CMMs, vision systems, wax
trace and Genspect; the plugin was then built against a docstring naming
calipers, micrometers and thread gages. Those are not the same population, and
the difference is the whole decision: bay-mounted capital equipment with a
controlling PC on one side, hand tools in a drawer on the other.

Sixteen months on, measuringtools holds zero rows against 204 machines. The
metrology inventory waiting to be entered is 134 instruments - 35 CMM, 32 form
tracer, 23 vision, 18 wax, 14 measuring, 8 Genspect, 2 eddy, 1 spline - every
one bay-mounted with a controlling PC, and not one hand gage among them. CMM is
already a machinetype, and Part Marker is already a machine while Form Tracer
is a measuring tool, so the line is already drawn somewhere no operator can
predict.

Retires the plugin and the measuring_tool asset type. Calibration is not lost:
it moves onto machines as four nullable columns with status still derived at
read, which is a gain rather than a port because a machine could not record
calibration at all before. ADR-005's one concrete objection - that "show me all
measuring tools" would need an enumeration of type names - is answered with
machinetypes.ismetrology, so it stays a single-column filter.

Taken now because measuringtools is empty. After the 134 rows are entered they
carry relationships, audit history, map positions and collector links, and the
migration stops being a drop.

Scope: product

docs/adr/ADR-018-metrology-instruments-are-machines.md

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+# ADR-018: Metrology instruments are machines
+
+- Status: ACCEPTED
+- Date: 2026-09-04
+- Deciders: ShopDB maintainers
+- Relates to: ADR-001 (asset as the platform contract), ADR-006 (collector
+  contract), ADR-008 (a plugin owns its tables and its migration chain)
+- Supersedes: ADR-005 (equipment plugin scope vs measuringtools plugin)
+
+## Context
+
+ADR-005 split manufacturing machinery from metrology instruments into two
+plugins on the grounds that they are two asset classes: machines make parts,
+measuring tools measure them. It named the metrology population as "CMMs,
+Keyence vision systems, wax-and-trace surface profilometers, GenSpec
+instruments", and the `measuringtools` plugin was built against a docstring
+naming a different population again: "calipers, micrometers, thread gages,
+bore gages, height gages".
+
+Those two lists are not the same thing, and the difference is the whole
+decision. One is bay-mounted capital equipment with a controlling PC. The other
+is hand tools in a drawer. The plugin was built for the second and populated
+with neither.
+
+Sixteen months on, the evidence is in:
+
+- `measuringtools` holds **zero rows**, against 204 machines, 305 computers,
+  73 network devices and 53 printers.
+- The metrology inventory actually waiting to be entered is 134 instruments:
+  35 CMM, 32 form tracer, 23 vision, 18 wax trace, 14 measuring, 8 Genspect,
+  2 eddy current, 1 spline. Every one is bay-mounted with a controlling PC.
+- Not one caliper, micrometer, thread gage, bore gage or height gage. The
+  population the plugin's own docstring justifies it with does not exist at
+  the reference site and is not planned.
+- `CMM` already exists as a `machinetype`, and `Part Marker` is already
+  classed as a machine while `Form Tracer` is classed as a measuring tool.
+  Two bay instruments with a controlling PC, split across two plugins, with
+  the line drawn somewhere no operator can predict.
+
+The split has therefore been paying an ongoing cost - a duplicate type page, a
+settings group, a nav section, a migration chain, a searcher, dashboard cards,
+map colours, label printing and four collector `pctypemap` entries - to model a
+distinction that the inventory does not make.
+
+## Decision
+
+Metrology and inspection instruments are machines. The `measuringtools` plugin
+is retired and the `measuring_tool` asset type is removed.
+
+### Calibration moves to machines, and is not lost
+
+`machines` gains the calibration lifecycle as nullable columns:
+
+```
+machines
+- calibrationintervaldays   integer, null
+- lastcalibrationdate       date, null
+- nextcalibrationdate       date, null
+- calibrationprovider       varchar(150), null
+```
+
+Calibration status stays **derived at read time** from `nextcalibrationdate`
+(`overdue` / `duesoon` / `current` / `unknown`, 30-day window), exactly as
+`measuringtools` derived it and for the same reason: a stored status is stale
+the day after it is written. The helper moves across unchanged.
+
+This is a net gain rather than a port. A machine could not record calibration
+at all before, and a part marker with a certified fixture, or a machine with a
+calibrated probe, had nowhere to put it.
+
+### `machinetypes.ismetrology` answers ADR-005's objection
+
+ADR-005 made one concrete technical argument, and it deserves a concrete
+answer rather than silence:
+
+> "show me all measuring tools" requires an enumeration of measuring-instrument
+> equipmenttype values, which scales badly.
+
+That is correct about enumeration and it is why `machinetypes` gains a boolean
+`ismetrology`. The metrology view stays a first-class single-column filter -
+`WHERE mt.ismetrology = 1` - not a hardcoded list of type names that drifts the
+first time someone adds an instrument type. The calibration report, the gage
+lab view and any compliance export all hang off that flag.
+
+### What does not change
+
+- `assets.gaugelabreference` stays on the core asset. The gage tag was never a
+  `measuringtools` column, so it survives untouched and stays searchable for
+  every asset type.
+- The collector's device identity file and the `measuringtoolid` payload alias
+  stay accepted. Bays staged with `measuringtool-id.txt` keep reporting; only
+  the asset type they resolve to changes.
+- Instrument types keep their names. A Form Tracer is still a Form Tracer, now
+  as a `machinetype` with `ismetrology = 1`.
+
+## Consequences
+
+**The migration is free today and expensive later.** `measuringtools` has zero
+rows, so retirement is a `DROP TABLE`, not a data migration. Once the 134
+staged instruments are entered they carry relationships, audit history, map
+positions and collector links, and every one of those would have to be
+rewritten. This ADR is being taken now specifically because the window is open.
+
+**A site that did populate it needs a real migration.** The reference site did
+not and the sister site shows no sign of having done so - its branch contains
+no deliberate work on the plugin in its entire history - but that is inference,
+not a query against their database. Confirm before the removal ships to another
+site. If rows exist, the retirement migration must copy them into `machines`
+with `ismetrology` types rather than drop them.
+
+**Four collector `pctypemap` entries change asset type.** `gea-shopfloor-cmm`,
+`-keyence`, `-genspect` and `-waxtrace` resolve to `machine` instead of
+`measuring_tool`. The `collector:measuringtool` relationship label is kept as a
+recognised origin marker so links a collector already created are still
+archived correctly rather than orphaned.
+
+**One fewer of everything.** A duplicate type page, a settings group, a plugin
+migration chain, a gated searcher, a set of dashboard cards, a map colour entry
+and a label-printing branch all go. That is the point.
+
+**ADR-005 is superseded, not reversed in principle.** Its reasoning was sound
+for the population it described. The population never arrived. If a site ever
+does track hand gages in quantity, the answer is a `handgages` plugin against
+that real inventory - not the reinstatement of this one.