Generate thin model classes over a hand-written runtime

[OmarAlJarrah]

Problem

Inlining forward-compat machinery (additionalProperties, validate/validity, dual accessors) into every generated class doesn't scale — the reference SDK is ~850k lines across ~1,100 model files, which would blow our coverage floor and binary-compat baseline.

Proposed change

Push the invariant machinery into the runtime (the four-state field model) and emit only the field list + accessors per model (≈<100 lines each). Decide up front to exclude generated modules from the aggregate Kover floor and give them a separate .api baseline.

Acceptance

• Generated models are thin
• Coverage + binary-compat strategy for generated code documented

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
• Related additionalProperties pass-through on generated models #52, Generate unions as private-ctor + per-variant accessors + visitor (Java-8 safe) #53, Generate forward-compatible enums (open value + known/value pair) #54, Generate discriminator/const fields as defaulted raw values with dual accessors #65 — field / union / enum / discriminator templates feed thin models

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Priority: highest · Effort: large