Proposal: metric-mm measurement system (millimetre labels)

[archicorp46-debug]

Construction drawings in Korea (and architecture practice broadly in JP/EU) are dimensioned in millimetres, but formatMeasurement currently supports metric (metres) and imperial only — so embedders have to overlay their own readouts to show mm, duplicating the built-in measurement pills.

Proposal: a metric-mm measurement system that renders lengths as integer millimetres (e.g. 18400 mm instead of 18.4 m):

• formatMeasurement: metric-mm branch — Math.round(m * 1000) + mm suffix
• settings surface: expose the new option wherever measurementSystem is chosen
• no behaviour change for existing metric/imperial users

If this direction works for you, we're happy to send the PR (with unit cases for 0.2 m → 200 mm, 18.4 m → 18400 mm, and 0.5 mm rounding).

[Aymericr]

Millimetre labels are a valid direction, but the current implementation boundary is broader than formatMeasurement: the persisted useViewer.unit value also drives numeric editor inputs, floor-plan export/calibration, area and volume formatting, and the public registry GeometryContext.unit contract. Adding a third union member without an explicit semantic decision would leave some controls in metres while their in-world pills switch to millimetres.

For a production-ready change, please define one choice first:

1. Label-only format: keep metric editor inputs and m²/m³ unchanged, add a separate linear-label format used by measurement pills/guides/exports. This is the narrower and safer first PR.
2. Full metric-mm unit system: convert every linear input/control to millimetres too, while explicitly defining area/volume behavior and migrating the persisted preference.

I recommend option 1 for the first slice. Acceptance should cover 0.2 m → 200 mm, 18.4 m → 18400 mm, negative values, the 0.5 mm rounding boundary, preference persistence, and the 2D/3D measurement label paths. Keeping the issue open for that scoped implementation.