Add photons_absorbed, the inverse of the sensor signal/expose

[roytsmart]

Adds photons_absorbed to the sensor materials and to ImagingSensor, mapping measured electrons back into the number of absorbed photons.

Materials

AbstractSensorMaterial.photons_absorbed(electrons, wavelength, direction=1) inverts the noiseless signal. Because the sensor absorbance is folded into the effective area, signal runs at unit absorbance, so the inverse depends only on the quantum yield and the charge collection efficiency, not the absorbance (it is photons_absorbed, not photons_incident). IdealSensorMaterial and the silicon materials each supply a concrete implementation.

A new test_photons_absorbed checks that it exactly recovers the photons passed through signal at unit absorbance, for both ideal and silicon materials and for normal/oblique incidence.

Sensor

ImagingSensor.photons_absorbed mirrors expose: it inverts the material response and divides by the exposure time, giving the incident photon rate.

Why

This is the detector-response inverse needed to make optika.systems.LinearSystem.image/backproject a matched forward/transpose pair (that wiring lives in the interpolated-system branch, #152).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.54%. Comparing base (bf75a1f) to head (2fcd39e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #191   +/-   ##
=======================================
  Coverage   99.54%   99.54%           
=======================================
  Files         116      116           
  Lines        6374     6414   +40     
=======================================
+ Hits         6345     6385   +40     
  Misses         29       29           

Flag	Coverage Δ
unittests	99.54% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.