Codegen: implement target emission and document the pipeline

[hyperpolymath]

Problem

src/codegen/* (consent.rs, accessibility.rs, i18n.rs, parser.rs) are stubs — the generate pipeline does not yet emit target-language code end-to-end. The README/decorator reference implies a working pipeline, which slightly over-claims current capability.

Scope

• Implement target-language emission for @consent, @accessible, @i18n decorators.
• Wire manifest -> validate -> generate end-to-end with a real example under examples/compliant-app/.
• Add docs/developer/CODEGEN.adoc documenting the stages (parse → ABI validation → emit) and extension points (how to add a decorator).
• Align README claims with actual capability (or add a "current status" note).

Context

The ABI (Idris2) and FFI (Zig) layers are real and tested; codegen is the missing piece for the MVP (see .machine_readable/6a2/STATE.a2ml route-to-mvp).