feat: redesign Solution display — single tree, conditional fields, CTBase palette

[ocots]

Summary

Redesigns the Solution REPL display for clarity and consistency with the CTBase palette conventions.

Before

• Solver:
  ✓ Successful  : true
  │  Status     : Solve_Succeeded
  │  Message    : Solve_Succeeded
  │  Iterations : 10
  │  Objective  : 0.5
  └─ Constraints violation : 1.0e-9

• Variable: v = 1.5

• Boundary duals: [1.2, -0.8]

After

Single tree, no bullet points, fields omitted when not provided:

Solution  ✓ successful
  │  Objective : 0.5
  │  T : 1.5
  │  ├─ dual lb : [0.0]
  │  └─ dual ub : [0.3]
  │  Boundary duals : [1.2, -0.8]
  │
  │  Iterations : 10
  │  Status : first_order
  │  Message : optimal
  └─ Constraints violation : 1.0e-9

Flow result (no solver metadata):

Solution  ✓ successful
  │  Objective : 0.5
  └─ Message : Solution computed by CTFlows OCP flow

Changes

src/Solutions/solution_types.jl

• SolverInfos: iterations, status, message, constraints_violation are now Union{T, Core.NotProvidedType} instead of mandatory concrete types.

src/Solutions/build_solution.jl

• Both build_solution overloads: the four fields above default to Core.NotProvided.
• Accessor return-type annotations removed (they now return Union{T, NotProvidedType}).

src/Solutions/show.jl

• Complete rewrite: single tree using │/└─/├─, fmt.muted for structural chars, fmt.label/fmt.value for content.
• Empty │ separator appears between primal data (objective, variable, duals) and solver metadata (iterations, status, message, cv) when both sections are non-empty.
• NotProvided fields are silently omitted.

Tests

• test_solution_show.jl: 4 test cases, including a NotProvided-omission case.
• test_empty_dual_model.jl: "Solver" → "Solution" string check.

docs/src/solution/overview.md

• New "Displaying a solution" section with 4 executable @repl examples.

Related

• CTFlows cleanup tracked in: Remove sentinel values from build_solution call (requires CTModels beta release) CTFlows.jl#323