Add support for state-dependent running costs

[goerz]

Adds make_xi and J_b functions in QuantumControl.Functionals, with support for automatic differentiation.

This is a pre-requisite for state-dependent-running costs in GRAPE, see JuliaQuantumControl/GRAPE.jl#53

[codecov]

Codecov Report

❌ Patch coverage is 91.89189% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.5%. Comparing base (8891bab) to head (0952feb).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/functionals.jl	91.6%	5 Missing ⚠️
ext/QuantumControlZygoteExt.jl	87.5%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #103     +/-   ##
========================================
+ Coverage    88.3%   88.5%   +0.3%     
========================================
  Files          20      20             
  Lines        1046    1120     +74     
========================================
+ Hits          923     991     +68     
- Misses        123     129      +6     

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[Copilot]

Pull request overview

Adds the missing building blocks in QuantumControl.Functionals for state-dependent running costs (as needed by GRAPE): a running-cost functional J_b and a corresponding state-gradient builder make_xi, with AD backend support and tests.

Changes:

• Add make_xi (analytic/automatic selection, default AD framework support) and J_b to QuantumControl.Functionals.
• Implement make_automatic_xi for Zygote and FiniteDifferences via package extensions.
• Add comprehensive tests for make_xi and a basic integration test for J_b; add a Documenter interlink fallback label.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/functionals.jl	Introduces make_xi/make_automatic_xi and J_b, plus exports and storage access.
ext/QuantumControlZygoteExt.jl	Adds Zygote-based automatic differentiation path for xi.
ext/QuantumControlFiniteDifferencesExt.jl	Adds FiniteDifferences-based automatic differentiation path for xi.
test/test_functionals.jl	Adds tests for make_xi modes/backends and a J_b integration test.
docs/make.jl	Adds an interlinks fallback label referenced by J_b docs.

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