fix: off-grid laser lines, dropped emission sample, and QE without an emission filter

[tlambert03]

A few small spectral bugs I found while working on the confocal excitation fix (coming in a follow-up PR):

• Spectrum * Spectrum zero-filled any wavelength present in only one of the two spectra, so combining spectra on different grids silently lost signal. e.g. FITC with a detector QE sampled every 2 nm gave half the photons it should (53 vs 107 photons/s), and two 1 nm grids offset by 0.5 nm gave exactly 0. Both are now interpolated onto the union of their wavelengths (new Spectrum.at()).
• similarly, a laser between the 1 nm samples of the absorption spectrum (e.g. 488.5 nm) gave exactly zero excitation. The cross-section is now interpolated at the light's wavelengths, and excitation filters are applied with the new Spectrum.transmitted() (so a laser line stays a laser line).
• bin_spectrum dropped the lowest-wavelength sample (right-closed bins), so PSF weights were missing a bit of the emission. widefield and confocal images are now ~0.3-1.5% brighter in a fluorophore's main channel (more, relatively, for weak bleedthrough).
• detector QE wasn't applied to channels without an emission filter.

Also adds OpticalConfig.detected_fraction() and Fluorophore.emission_per_absorbed_photon. filtered_emission_rate now spans the fluorophore's full emission range (zeros outside the filter) and absorption_rate the light's wavelengths; totals are unchanged.

[codecov]

Codecov Report

❌ Patch coverage is 97.29730% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.26%. Comparing base (8362e0f) to head (8fefa3e).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/microsim/schema/optical_config/config.py	95.83%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #161      +/-   ##
==========================================
+ Coverage   88.15%   88.26%   +0.10%     
==========================================
  Files          48       48              
  Lines        3292     3314      +22     
==========================================
+ Hits         2902     2925      +23     
+ Misses        390      389       -1     

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