refactor: co-locate each strategy's contract with its abstract type

[ocots]

Summary

Behavior-preserving reorganization so every strategy family keeps its contract
stub next to its abstract type, mirroring the existing DOCP/contract.jl
precedent. Before this PR the contract was co-located only for the discretizer;
the modeler and solver contracts were scattered.

Strategy	Abstract type	Contract	Now lives in
Discretizer	AbstractDiscretizer	discretize	DOCP/contract.jl (already)
Modeler	AbstractNLPModeler	build_model/build_solution	Modelers/contract.jl (new)
Solver	AbstractNLPSolver	solve(nlp, ::AbstractNLPSolver)	Solvers/contract.jl (new)

Changes

• Modelers: new contract.jl with the canonical build_model/build_solution
NotImplemented stubs, narrowed to ::AbstractNLPModeler (documented with
  $(TYPEDSIGNATURES)). Optimization keeps bare generic declarations in
  building.jl so the binding exists and the re-export stays valid.
• Solvers: split common_solve_api.jl into contract.jl (mid-level
  solve(nlp, ::AbstractNLPSolver) stub + __display) and orchestration.jl
  (high-level composition).
• DOCP: merged types.jl + accessors.jl into discretized_model.jl;
  renamed building.jl → conveniences.jl.
• Docs: resynced the per-submodule source-file lists in docs/api_reference.jl
  (also fixes pre-existing staleness: adds built_model.jl, moves solver_info.jl
  under Solvers, drops removed contract_impl/builders/common_solve_api).
• Tests: FakeModeler now subtypes AbstractNLPModeler so an unimplemented
  (problem, modeler) pair still yields NotImplemented.

Behavior

Only semantic change: the modeler stub narrows from an untyped 3rd arg to
::AbstractNLPModeler (still NotImplemented; more correct). Concrete CTDirect
methods are strictly more specific → no ambiguity; CTDirect needs no source
change.

Tests

Full CTSolvers suite green (non-extension suites + extensions + Aqua). CTDirect
smoke is currently blocked by an unrelated CTBase 0.25↔0.26-beta compat
drift, not by this reorg.

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