[ocots]

Closes #965.

Decision: re-export none of the other initial-guess helpers

build_initial_guess stays the only CTModels initial-guess function exported by OptimalControl. initial_guess, pre_initial_guess, validate_initial_guess, initial_state, initial_control, initial_variable and PreInitialGuess stay unexported. Reasons, helper by helper:

Helper	What it does	Why not exported
validate_initial_guess(ocp, ig)	checks dimensions of an InitialGuess	build_initial_guess(ocp, data) already validates: it is "the single entry point that guarantees a validated initial guess" (its docstring). Checking a guess before solving = building it. A second name for the same check adds surface, not capability.
initial_guess(ocp; state, control, variable)	builds an unvalidated guess	covered by @init and by the named tuple (state=…, control=…, variable=…) given to build_initial_guess / solve, both validated. Also, the name equals the solve keyword initial_guess: an exported function of that name would invite confusion with initial_guess = … in user scripts.
pre_initial_guess, PreInitialGuess	an internal intermediate before dimensions are known	no use case a named tuple does not cover.
initial_state, initial_control, initial_variable	per-component builders used by initial_guess	implementation details. initial_state reads as "x(t₀)" for a user, which it is not. Inspection is covered by state(ig), control(ig), variable(ig).

Users who still want one of them can call it qualified (CTModels.Init.validate_initial_guess): nothing is hidden, only not exported.

Changes

• Docs, page "Initial guess": the section "Inspect a guess" becomes "Inspect or check a guess". It now shows that build_initial_guess raises an IncorrectArgument on a component of the wrong dimension, so it is the way to check a guess before solving. The example is executed and checked by a hidden @assert.
• Test, test/suite/reexport/test_ctmodels.jl: the seven names are asserted not exported. The decision is then recorded in the suite, and a future re-export has to update this test on purpose.
• CHANGELOG entry under [Unreleased].

Checks

• suite/reexport/test_ctmodels.jl: 325/325 passing.
• The doc example, run by hand: build_initial_guess(ocp2, (state=[-1.0, 0.0, 0.0],)) raises IncorrectArgument: Initial state dimension mismatch (got vector of length 3, expected 2).
• Docs build not run locally.

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