Add touchpad positional accuracy example

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A touchpad is tested by pressing it: a robot puts a known force on a known coordinate, and the pad reports where it thinks it was touched. The number that matters is the distance between the two.

touchpad_ptp/ records that distance against the Precision Touchpad linearity limits — 0.5 mm across the pad, relaxed to 1.5 mm within 3.5 mm of an edge, where the sensor is least linear. Windows reports these distances in himetric units (0.01 mm), which is what the HID read in plugs/touch_robot.py converts from.

The grid is one measurement, not nine

A manual test taps each target and writes down whether it looked right. Here each grid becomes a curve indexed by target, with max validated against the spec limit and mean against a tighter working limit.

max is what fails the unit — one target outside the limit is a failure no average should absorb. Keeping the whole series alongside it separates a pad that is off in one corner from one that is off everywhere, which is the difference between a fixture problem and a sensor problem. The worst edge target is recorded a second time as a scalar, so the failing number is filterable on its own and trendable across a population.

Two limits, one press

The central grid and the edge band run the same code against the same fixture. They are separate phases only because the spec allows three times the error inside the border strip, and a single limit would either pass a bad centre or fail a good edge.

Click actuation force comes off the same fixture in the same cycle. It has nothing to do with position, but a dome that stiffens or softens is an early sign of the same mechanical drift that moves the positional error.

Plugs return representative data so the procedure runs anywhere; each method keeps its real call commented directly above — motion controller over VISA for the press, HID digitizer report for the reported contact, load cell triggered on switch closure for the click.

Verified with tofupilot run ./touchpad_ptp — all four phases PASS, aggregation validators firing with computed values.