[ocots]

Closes #467.

Stacked on #463 (switching structure): both edit ext/CTModelsJSON.jl, so the base of this PR is feat/switching-structure. Once #463 is merged, the base moves to main.

What changes

JSON3.jl is deprecated in favour of JSON.jl v1. The JSON format of a Solution now uses it.

• Dependency. JSON3 → JSON (weak dependency, test dependency, [compat], docs environment). The extension keeps its name, CTModelsJSON, which now matches its trigger.
• Extension.
  • JSON3.read(s) → JSON.parse(s; dicttype=Dict{String,Any}): plain dictionaries, read with blob["key"]; _to_plain is no longer needed.
  • JSON3.pretty → JSON.json(...; pretty=true).
  • An export serializes first and writes after, so a value JSON cannot hold (a NaN, as before) leaves no half-written file.
• Tag. JSON3Tag → JSONTag, and the stubs raise ExtensionError(:JSON; ...). This is a breaking change, accepted in favour of clean naming. format=:JSON is unchanged; BREAKING.md has the migration block.
• File format. Unchanged (same keys, matrices as lists of rows).

Tests

• New test/suite/serialization/test_json_format.jl:
  • reads two files written by the JSON3-based version (test/fixtures/json3_*.json, exported with CTModels 0.19.8: one plain solution and one with duals);
  • checks that a file written now has the same keys and shapes as the previous one (only format_version differs, as announced in feat(Solutions): switching structure of a solution; strict time grids #463);
  • round trip, NaN rejected with no file left, JSONTag replaces JSON3Tag.
• The existing serialization tests are ported (JSON3.read/write → JSON.parse/json, JSONTag).
• Full suite: 4738 passed, then the serialization and meta tests again after the last edits (2377 passed).
• Docs: built in two passes; the only errors are the @extref to the CTBase.Plotting building blocks of #613 (not released yet), already there in feat(Solutions): switching structure of a solution; strict time grids #463.

Not in this PR

• The round trip of a Solution with unset optional fields (NotProvided) fails in both formats: Export/import round trip fails for a Solution with unset optional solver fields (NotProvided) #464. It is independent of the choice of the JSON package.
• OptimalControl documentation (using JSON3, docs/Project.toml, docs/make.jl): to update once this is released; issue to open there.

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