Expression Support (PR #215 rebased onto main)

[sshin23]

Summary

• Squash-merges PR Expression Support #215 (Expression Support by @hfytr) onto current main
• Adds pre-computed subexpression caching: expressions are evaluated once per obj/grad!/jac_coord!/hess_coord! call instead of re-evaluated on every reference
• Resolves conflicts with two-stage model, getter/setter API, and other recent changes

Changes from PR #215

• src/graph.jl: new Expression/AbstractNode subtype for subexpressions
• src/nlp.jl: ExaCore/ExaModel carry expression storage
• src/hessian.jl, src/jacobian.jl, src/gradient.jl: expression eval threading
• src/simdfunction.jl: SIMD kernel updates

🤖 Generated with Claude Code

[github-actions]

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julia --project=@runic -e "using Runic; exit(Runic.main(ARGS))" -- --fix <files>

(or git runic main if you have the git wrapper installed)

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[sshin23]

Closing — pushing changes directly to supersede PR #215 instead.