Migrate FormObjectToJSON to use visitor pattern

[darianmiller]

Once Delphi.Forms.Visitor (#41) ships, evaluate migrating the JSON
serialization in Delphi.Forms.JSON to use the visitor for traversal.

This migration is less straightforward than NormalizeForm or Statistics
because JSON serialization builds a nested TJSONObject tree that mirrors
the AST hierarchy -- the visitor's flat callback model does not naturally
produce nested output. A stack-based approach in the visitor (push/pop
JSON objects on enter/leave) can work but adds complexity.

Scope

• Evaluate whether the visitor pattern simplifies or complicates the
  JSON serialization code
• If beneficial: implement a TJsonVisitor with a JSON object stack
• If not beneficial: document the decision and close as won't-fix
• If migrated: all existing JSON tests pass with identical output
• FormFileToJSON and FormObjectToJSON public API unchanged

Acceptance criteria

• Decision documented (migrate or skip with rationale)
• If migrated: all JSON tests pass, public API unchanged
• If skipped: ticket closed with explanation