Update R package interface to use new XPtr API

[Andrea-Havron-NOAA]

Labels: refactor, R

Parent: #1644

Update all user-facing R code — fitting functions, helper utilities, vignettes, and tests — to use the new create_() / setter / CreateTMBModel() workflow instead of methods::new().

Tasks

• Update FIMSFrame fitting workflow
• Update all vignettes and example scripts
• Update testthat tests
• Verify describe_model() output matches expected model configuration in tests
• Rerun memory tests and profiles to compare with S4 implementation (regression test against Child Issue 1 (Establish memory baseline for current S4 interface #1645) baseline)