feat: add control-free optimal control problems support

- Add control-free problems documentation and examples
- Implement ExponentialGrowth and HarmonicOscillator test problems
- Fix error display for zero reference objectives (use absolute error)
- Make color thresholds consistent with test tolerances
- Add unique IDs to all section headings to avoid anchor conflicts
- Fix MD036 linting warnings in documentation
- All 84 tests passing successfully

BREAKING CHANGE: None (pure addition)

Workaround: Uses u(t) penalty term (1e-5*u(t)^2) instead of u(t)==0 constraint
Future: Remove u(t) declaration and penalty when native control-free syntax is implemented

Author: ocots

docs/make.jl

@@ -207,6 +207,7 @@ with_api_reference(src_dir, ext_dir) do api_pages
             "Basic Examples" => [
                 "Energy minimisation" => "example-double-integrator-energy.md",
                 "Time mininimisation" => "example-double-integrator-time.md",
+                "Control-free problems" => "example-control-free.md",
             ],
             "Manual" => [
                 "Define a problem" => "manual-abstract.md",

docs/src/api/private.md

@@ -0,0 +1,244 @@
+```@meta
+EditURL = nothing
+```
+
+# Private API
+
+This page lists **non-exported** (internal) symbols of `OptimalControl`.
+
+
+---
+
+### From `OptimalControl`
+
+
+## `DescriptiveMode`
+
+```@docs
+OptimalControl.DescriptiveMode
+```
+
+
+## `ExplicitMode`
+
+```@docs
+OptimalControl.ExplicitMode
+```
+
+
+## `SolveMode`
+
+```@docs
+OptimalControl.SolveMode
+```
+
+
+## `_DEFAULT_DISPLAY`
+
+```@docs
+OptimalControl._DEFAULT_DISPLAY
+```
+
+
+## `_DEFAULT_INITIAL_GUESS`
+
+```@docs
+OptimalControl._DEFAULT_INITIAL_GUESS
+```
+
+
+## `_INITIAL_GUESS_ALIASES`
+
+```@docs
+OptimalControl._INITIAL_GUESS_ALIASES
+```
+
+
+## `_INITIAL_GUESS_ALIASES_ONLY`
+
+```@docs
+OptimalControl._INITIAL_GUESS_ALIASES_ONLY
+```
+
+
+## `_ansi_color`
+
+```@docs
+OptimalControl._ansi_color
+```
+
+
+## `_ansi_reset`
+
+```@docs
+OptimalControl._ansi_reset
+```
+
+
+## `_build_components_from_routed`
+
+```@docs
+OptimalControl._build_components_from_routed
+```
+
+
+## `_build_or_use_strategy`
+
+```@docs
+OptimalControl._build_or_use_strategy
+```
+
+
+## `_build_partial_description`
+
+```@docs
+OptimalControl._build_partial_description
+```
+
+
+## `_build_partial_tuple`
+
+```@docs
+OptimalControl._build_partial_tuple
+```
+
+
+## `_build_source_tag`
+
+```@docs
+OptimalControl._build_source_tag
+```
+
+
+## `_complete_components`
+
+```@docs
+OptimalControl._complete_components
+```
+
+
+## `_complete_description`
+
+```@docs
+OptimalControl._complete_description
+```
+
+
+## `_descriptive_action_defs`
+
+```@docs
+OptimalControl._descriptive_action_defs
+```
+
+
+## `_descriptive_families`
+
+```@docs
+OptimalControl._descriptive_families
+```
+
+
+## `_determine_parameter_display_strategy`
+
+```@docs
+OptimalControl._determine_parameter_display_strategy
+```
+
+
+## `_explicit_or_descriptive`
+
+```@docs
+OptimalControl._explicit_or_descriptive
+```
+
+
+## `_extract_action_kwarg`
+
+```@docs
+OptimalControl._extract_action_kwarg
+```
+
+
+## `_extract_kwarg`
+
+```@docs
+OptimalControl._extract_kwarg
+```
+
+
+## `_extract_strategy_parameters`
+
+```@docs
+OptimalControl._extract_strategy_parameters
+```
+
+
+## `_has_complete_components`
+
+```@docs
+OptimalControl._has_complete_components
+```
+
+
+## `_print_ansi_styled`
+
+```@docs
+OptimalControl._print_ansi_styled
+```
+
+
+## `_print_component_with_param`
+
+```@docs
+OptimalControl._print_component_with_param
+```
+
+
+## `_route_descriptive_options`
+
+```@docs
+OptimalControl._route_descriptive_options
+```
+
+
+## `_unwrap_option`
+
+```@docs
+OptimalControl._unwrap_option
+```
+
+
+## `display_ocp_configuration`
+
+```@docs
+OptimalControl.display_ocp_configuration
+```
+
+
+## `get_strategy_registry`
+
+```@docs
+OptimalControl.get_strategy_registry
+```
+
+
+## `solve_descriptive`
+
+```@docs
+OptimalControl.solve_descriptive
+```
+
+
+## `solve_explicit`
+
+```@docs
+OptimalControl.solve_explicit
+```
+
+
+## `will_solver_print`
+
+```@docs
+OptimalControl.will_solver_print
+```
+