Declare setguess and ImperativeAffect as public API (#4669)

Mark the docstringed, user-facing `setguess` and `ImperativeAffect`
(both owned by ModelingToolkitBase) with `@public`. The `@import_mtkbase`
macro in ModelingToolkit then propagates this to the top-level
`ModelingToolkit` module, so downstream packages can drop their
ExplicitImports allow-list entries for these names.

Co-authored-by: ChrisRackauckas-Claude <accounts@chrisrackauckas.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: 3 people

lib/ModelingToolkitBase/src/ModelingToolkitBase.jl

@@ -345,12 +345,12 @@ const set_scalar_metadata = setmetadata
 @public unbound_outputs, is_bound
 @public AbstractSystem, CheckAll, CheckNone, CheckComponents, CheckUnits
 @public t, D, t_nounits, D_nounits
-@public SymbolicContinuousCallback, SymbolicDiscreteCallback
+@public SymbolicContinuousCallback, SymbolicDiscreteCallback, ImperativeAffect
 @public VariableType, MTKVariableTypeCtx, VariableBounds, VariableConnectType
 @public VariableDescription, VariableInput, VariableIrreducible, VariableMisc
 @public VariableOutput, VariableStatePriority, VariableUnit, collect_scoped_vars!
 @public collect_var_to_name!, collect_vars!, eqtype_supports_collect_vars, hasdefault
-@public getdefault, setdefault, iscomplete, isparameter, modified_unknowns!
+@public getdefault, setdefault, setguess, iscomplete, isparameter, modified_unknowns!
 @public renamespace, namespace_equations
 @public check_mutable_cache, store_to_mutable_cache!, should_invalidate_mutable_cache_entry
 @public convert_bindings_for_time_independent_system, get_w