Generate discriminator/const fields as defaulted raw values with dual accessors

[OmarAlJarrah]

Problem

Discriminator/const fields and the typed+raw dual-accessor pattern fall out of the field model and need a codegen template.

Proposed change

Generate const/discriminator fields as defaulted raw values, with a typed accessor and a raw accessor per field. (Two getters + two setters per field reinforces the separate binary-compat baseline for generated code.)

Acceptance

• Const/discriminator template
• Dual accessors generated

Depends on the four-state field model.

Dependencies

• Blocked by Design a dependency-free four-state JSON field model for generated types #50 — four-state JsonField model

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Priority: medium · Effort: small