Persist the machine clamp rail in %APPDATA%\OpenNest\clamps.json, in
the same shape as the nest defaults store: camelCase JSON written with
retry, and a read that never throws. Clamps are off by default: a
missing or unreadable file keeps them off, and nothing changes until
the settings are enabled with at least one center.
Each value is read independently; an invalid one (including a single
bad clamp center) is ignored and reported as Partial so the desktop
can warn that the rail may lack a physical clamp. Values carry their
units and convert between inches and millimeters for a new nest.
aj
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