Investigate OT-2/App updates behind custom-labware height errors

[Copilot]

Investigates why the wireless color-sensor custom labware (byu-vcl#33) fails OT-2 tip-length calibration with a "too tall" error (Opentrons/ot2#10), and determines which Opentrons OT-2 / App versions changed to cause it.

Finding

The error is a fixed OT-2 hardware geometry limit, not a software regression. The labware declares zDimension: 100 mm, but the pipette can only be raised to ≈96.3 mm above the deck, so the legacy motion planner raises LabwareHeightError.

• The ceiling is homePosition − Z_RETRACT_DISTANCE(2mm) + critical_point_z — all pipette/hardware constants, independent of the app/robot release.
• The _build_safe_height / LabwareHeightError check has existed since the 2020 motion-planning refactors, present in every v4→v9 build. This explains why downgrading to v8.8.1 behaved identically to v9 — no version omits the limit, so no downgrade can fix it.
• Schema/calibration-flow changes across v6→v9 were reviewed and ruled out (the labware is schema 2 and passes those checks; failure happens later during motion planning).

Adaptation

Reduce the loaded labware zDimension to ≤ ~95 mm and/or adopt the shorter replaceable-nozzle-on-fixed-body design; the tall 100 mm part cannot clear the gantry on any software version.

Changes

• docs/custom-labware-height-error-investigation.md — report with the exact error, root-cause trace (code citations into planning.py, pipette_handler.py, calibration/util.py, defaults_ot2.py), version history, the secondary mechanical (worn press-fit) issue, and four adaptation options.
• README.md — links the report under a new "Investigations" section.

Documentation-only; no hardware or code files modified.