Phase 6: IARC case print + solve IWR6843 mount gap

[dieterolson]

From hardware/build-plan.md Phase 6.

• Print IARC case v3 parts for the chosen display (cad/IARC_case/ - monitor_shell.stl vs Touch_Display2_*.stl variants). Note upstream PR #147 / merged #156 reworked Touch Display 2 parts.
• Assemble: Pi, OPS mount, sensor housing, display shell, feet
• Solve the IWR6843 mount gap: the IARC case STL set predates the IWR6843 (K-LD7 mounts only, no LEVM mount). Check upstream cad/ and issues first; if absent, design one. Constraints from the operator guide: board rotated so the vertical virtual array is physically vertical, antenna face unobstructed, rigid mount (small shifts appear as angle bias).
• Re-measure ALL geometry after mounting; re-run a short calibration session

A finished IWR6843LEVM mount design is a strong upstream contribution - every new builder will need one.