Unit overhaul 6/6: ExpressionComponent physical constants and parameter units

[henrikjacobsenfys]

Add physical constants and per-parameter units to ExpressionComponent: hbar (meV·s) and kb (meV/K) are provided automatically as read-only constants when their symbol appears in an expression; parameter_units sets a unit per parameter without rescaling; the expression's output unit is derived from the parameter units and x_unit and a warning is issued when it disagrees with y_unit; parameters are dimensionless by default.

Split out of the unit-system overhaul as an independent feature stacked on the core rename (#224) and the FitBinding redesign (#226).

Also advances components.ipynb to demonstrate the ExpressionComponent parameter_units API; the tutorial notebooks run green under tutorial-tests on this branch.

All unit tests pass.

---

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)

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 98.61111% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.48%. Comparing base (508ba38) to head (4919c02).

Files with missing lines	Patch %	Lines
...cs/sample_model/components/expression_component.py	98.61%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@                    Coverage Diff                    @@
##           split/02d-fit-binding     #227      +/-   ##
=========================================================
+ Coverage                  98.28%   98.48%   +0.20%     
=========================================================
  Files                         53       53              
  Lines                       3897     4037     +140     
  Branches                     660      697      +37     
=========================================================
+ Hits                        3830     3976     +146     
+ Misses                        39       33       -6     
  Partials                      28       28              

Flag	Coverage Δ
unittests	98.48% <98.61%> (+0.20%)	⬆️

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

Files with missing lines	Coverage Δ
...cs/sample_model/components/expression_component.py	99.15% <98.61%> (+1.23%)	⬆️

... and 1 file with indirect coverage changes

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