Implement cross-reactor Jacobian terms for preconditioners

[speth]

Changes proposed in this pull request

• Add cross-reactor terms to Jacobians for IdealGasMoleReactor and IdealGasConstPressureMoleReactor.
• Add ExtensibleReactor interface for setting Jacobian elements (get_jacobian_elements)
• Add ReactorNet.jacobian and ReactorNet.finite_difference_jacobian accessors for full-network Jacobians (in contrast to just the reactor-local parts available on each Reactor

If applicable, fill in the issue number this pull request is fixing

• Supersedes Extensible jacobian and network jacobian interfaces #1634
• Closes Addition of Jacobian Preconditioning Functionality to Cantera. enhancements#47

AI Statement (required)

• Extensive use of generative AI.
  Significant portions of code or documentation were generated with AI, including logic and implementation decisions. All generated code and documentation were reviewed and understood by the contributor. Implemented using Claude Code (Sonnet 4.6) and OpenAI Codex (ChatGPT-5.5).

Checklist

• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows contributing guidelines
• AI Statement is included
• The pull request is ready for review

[codecov]

Codecov Report

❌ Patch coverage is 82.83582% with 46 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.75%. Comparing base (4be08e6) to head (0ce9b23).
⚠️ Report is 21 commits behind head on main.

Files with missing lines	Patch %	Lines
src/zeroD/FlowDevice.cpp	67.85%	6 Missing and 3 partials ⚠️
src/zeroD/IdealGasMoleReactor.cpp	89.55%	2 Missing and 5 partials ⚠️
src/zeroD/flowControllers.cpp	63.15%	3 Missing and 4 partials ⚠️
interfaces/cython/cantera/delegator.pyx	66.66%	5 Missing ⚠️
src/zeroD/IdealGasConstPressureMoleReactor.cpp	91.11%	1 Missing and 3 partials ⚠️
src/zeroD/Reactor.cpp	63.63%	0 Missing and 4 partials ⚠️
include/cantera/zeroD/ReactorBase.h	66.66%	3 Missing ⚠️
include/cantera/zeroD/Wall.h	0.00%	3 Missing ⚠️
include/cantera/base/Delegator.h	71.42%	1 Missing and 1 partial ⚠️
include/cantera/zeroD/FlowDevice.h	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2113      +/-   ##
==========================================
+ Coverage   77.69%   77.75%   +0.06%     
==========================================
  Files         452      452              
  Lines       53203    53467     +264     
  Branches     8864     8920      +56     
==========================================
+ Hits        41335    41574     +239     
- Misses       8867     8879      +12     
- Partials     3001     3014      +13     

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[ischoegl]

I believe that there should be one sample file per application. I.e.., my preference would be to rename the pre-existing file to preconditioned_ignition.py and add new content as preconditioned_network.py or similar.