touch_stress sensor not in plugins but in the README

[rotematari]

The feature, motivation and pitch

Summary

plugin/sensor/README.md documents a Touch Stress sensor (mujoco.sensor.touch_stress) in detail — example XML, parametrization, result images — and links it to touch_stress.h. But the plugin does not exist in the source tree, and using it fails to compile the model.

Evidence (current main)

• plugin/sensor/register.cc registers only TouchGrid::RegisterPlugin() — no TouchStress.
• plugin/sensor/CMakeLists.txt compiles only register.cc, touch_grid.h, touch_grid.cc — no touch_stress.cc.
• The README link [Touch Stress](touch_stress.h) is dead — no touch_stress.{h,cc} in plugin/sensor/.
• doc/changelog.rst never mentions touch_stress.
• No PR or issue references it.

Steps to reproduce (MuJoCo 3.8.1, Python)

import mujoco
xml = '<mujoco><extension><plugin plugin="mujoco.sensor.touch_stress"/></extension></mujoco>'
mujoco.MjModel.from_xml_string(xml)
# -> XML Error: plugin mujoco.sensor.touch_stress not found

### Alternatives

_No response_

### Additional context

_No response_