Support logistic resources in Newton steady solver

[gustavdelius]

Summary

• add exported steady-resource companions for semichemostat and logistic dynamics
• resolve steady_<resource_dynamics>() generically when constructing the Newton residual
• support the positive logistic equilibrium branch and report depleted branches clearly
• document the extension convention and regenerate help pages and guides
• add companion, logistic integration, and regression tests

Testing

• MIZER_TEST_EXPERIMENTAL=true Rscript --vanilla -e 'devtools::load_all(); devtools::test(filter = "resource_logistic|resource_semichemostat|steadyState|steadyNewton")'
• 248 passed, 0 failed, 0 warnings, 0 skipped