feat: Add extended Optimize APIs (model callback, initial values)

[toolCHAINZ]

Summary

Two missing APIs on the Optimize solver: a callback for intermediate improving models
(enabling early termination and progress reporting), and warm-starting with an initial
variable assignment.

Z3 APIs

• Z3_optimize_register_model_eh — register a callback invoked each time the optimizer
  finds a new model that improves the objective
• Z3_optimize_set_initial_value — set an initial value for a variable to warm-start
  the optimization search

Notes

• register_model_eh is a callback API; the Rust wrapper should accept a closure and
  manage the lifetime of the callback data relative to the Optimize object
• The callback receives the current model, enabling the caller to extract intermediate
  solutions or terminate early via Z3_interrupt
• set_initial_value mirrors the analogous Z3_solver_set_initial_value (tracked in feat: Add solver control APIs (interrupt, next_split, initial values, solve_for) #537)
• Z3_optimize_translate is already covered by impl Translate for Optimize (merged in feat: impl Translate and Clone for Optimize #529)