Unit overhaul 5/6: Redesign FitBinding around FitTargets

[henrikjacobsenfys]

Replace FitBinding's parameter_name/modes API with a single targets spec that maps a model's declared predictions (its get_fit_targets()) onto keys of the parameters Dataset, and update ParameterAnalysis to consume the resolved FitTargets. Component models fit their single evaluate prediction; diffusion models select named predictions (area/width, plus delta_area for DeltaLorentz) with optional dataset-key overrides.

Split out of the unit-system overhaul as an independent, self-contained change. Builds on FitTarget (#221) and the get_fit_targets() model API added in the core rename (#224).

Also advances the tutorial notebooks to the new FitBinding(targets=…) API (tutorial0_basics, tutorial0_more_advanced, tutorial1_brownian); they run green under tutorial-tests on this branch.

All unit tests pass.

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Stack — splitting the monolithic "Unit system overhaul" (#217) into a reviewable series. Merge bottom-up:

1. Add unit-conversion utilities and FitTarget → develop (Unit overhaul 1/6: Add unit-conversion utilities and FitTarget #221)
2. Centralize Q-index validation via verify_Q_index → split/01-unit-utils (Unit overhaul 2/6: Centralize Q-index validation via verify_Q_index #222)
3. Store Q as a scipp Variable in 1/angstrom → split/02a-verify-q-index (Unit overhaul 3/6: Store Q as a scipp Variable in 1/angstrom #223)
4. Separate model unit into x_unit/y_unit + tutorials → split/02b-q-variable (Unit overhaul 4/6: Separate unit into x_unit/y_unit and update tutorials #224)
5. Redesign FitBinding around FitTargets → split/02c-unit-rename (Unit overhaul 5/6: Redesign FitBinding around FitTargets #226)
6. ExpressionComponent physical constants + parameter units → split/02d-fit-binding (Unit overhaul 6/6: ExpressionComponent physical constants and parameter units #227)

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[codecov]

Codecov Report

❌ Patch coverage is 97.67442% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.99%. Comparing base (d635379) to head (9cadb44).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/easydynamics/analysis/fit_binding.py	95.45%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #226      +/-   ##
===========================================
+ Coverage    98.81%   98.99%   +0.18%     
===========================================
  Files           53       53              
  Lines         3892     3897       +5     
  Branches       664      660       -4     
===========================================
+ Hits          3846     3858      +12     
+ Misses          23       18       -5     
+ Partials        23       21       -2     

Flag	Coverage Δ
unittests	98.99% <97.67%> (+0.18%)	⬆️

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

Files with missing lines	Coverage Δ
src/easydynamics/analysis/parameter_analysis.py	99.51% <100.00%> (+1.13%)	⬆️
src/easydynamics/analysis/fit_binding.py	95.23% <95.45%> (+0.36%)	⬆️

... and 1 file with indirect coverage changes

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